C#-Code

using System;
using System.Collections;
using System.Collections.Generic;

namespace Freitag_der_13_Tagesbestimmung
{
    class Program
    {
        static void Main(string[] args)
        {
            DateTime dtBeginn = new DateTime(0);
            DateTime dtNow = DateTime.Now;
            List<object[]> matches = new List<object[]>();
            Hashtable htAnzahl = new Hashtable();
            htAnzahl.Add(DayOfWeek.Monday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Tuesday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Wednesday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Thursday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Friday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Saturday.ToString(), 0);
            htAnzahl.Add(DayOfWeek.Sunday.ToString(), 0);

            while (dtBeginn < dtNow)
            {
                if (dtBeginn.Day == 13)
                    matches.Add(new object[] { dtBeginn.Day, dtBeginn.Month, dtBeginn.Year, dtBeginn.DayOfWeek });
                dtBeginn = dtBeginn.AddDays(1);
            }

            matches.ForEach(x => {
                //Console.WriteLine(string.Format(x[0].ToString() + "." + x[1].ToString() + "." + x[2].ToString() + ":\t" + x[3].ToString()));
                htAnzahl[x[3].ToString()] = (int)htAnzahl[x[3].ToString()] + 1;                
            });

            Console.WriteLine();
            Console.WriteLine(DayOfWeek.Monday.ToString() + ":   \t" + htAnzahl[DayOfWeek.Monday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Tuesday.ToString() + ":\t" + htAnzahl[DayOfWeek.Tuesday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Wednesday.ToString() + ":\t" + htAnzahl[DayOfWeek.Wednesday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Thursday.ToString() + ":\t" + htAnzahl[DayOfWeek.Thursday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Friday.ToString() + ":   \t" + htAnzahl[DayOfWeek.Friday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Saturday.ToString() + ":\t" + htAnzahl[DayOfWeek.Saturday.ToString()].ToString());
            Console.WriteLine(DayOfWeek.Sunday.ToString() + ":   \t" + htAnzahl[DayOfWeek.Sunday.ToString()].ToString());

            Console.ReadKey();
        }
    }
}

C#-Code

using System;
using static System.Console;

namespace freitag13 {

    class Program    {

        static void Main()        {
            int startjahr = 1, endjahr = 2017, startmonat = 1, endmonat = 12;
            int[] verteilung = new int[7];
            for (int jahr = startjahr; jahr <= endjahr; jahr++)
                for(int monat = startmonat; monat <= endmonat; monat++)  {
                    DateTime value = new DateTime(jahr, monat, 13);
                    verteilung[(int)value.DayOfWeek]++;
                }
            for (var i = 0; i < verteilung.Length; i++)
                WriteLine(i.ToString() + ": " + Enum.GetName(typeof(DayOfWeek), i).ToMyString(10) + " " + verteilung[i].ToString("n0"));
            ReadKey(true);
        }
    }

    static class MyExtensions    {

        public static string ToMyString<T>(this T s, int chars)        {
            string result = s.ToString();
            while (result.Length < chars)
                result += " ";
            return result;
        }
    }
}

C#-Code

    internal static class Program
    {
        private static void Main()
        {
            DateTime day = new DateTime(1, 1, 1);
            Time time = new Time
            {
                Monday = 0,
                Tuesday = 0,
                Wednesday = 0,
                Thursday = 0,
                Friday = 0,
                Saturday = 0,
                Sunday = 0
            };
            while (day.ToLongDateString() != DateTime.Now.ToLongDateString())
            {
                if (day.Day == 13)
                {
                    if (day.DayOfWeek == DayOfWeek.Monday) { time.Monday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Tuesday) { time.Tuesday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Wednesday) { time.Wednesday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Thursday) { time.Thursday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Friday) { time.Friday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Saturday) { time.Saturday += 1; }
                    if (day.DayOfWeek == DayOfWeek.Sunday) { time.Sunday += 1; }
                }
                Debug.WriteLine(day.ToShortDateString());
                day = day.AddDays(1);

            }
            Console.WriteLine("Seit unserer Zeitrechnung gab es:");
            Console.WriteLine($"{time.Monday} Montag den 13.");
            Console.WriteLine($"{time.Tuesday} Dienstag den 13.");
            Console.WriteLine($"{time.Wednesday} Mittwoch den 13.");
            Console.WriteLine($"{time.Thursday} Donnerstag den 13.");
            Console.WriteLine($"{time.Friday} Freitag den 13.");
            Console.WriteLine($"{time.Saturday} Samstag den 13.");
            Console.WriteLine($"{time.Sunday} Sonntag den 13.");
            Console.Read();
        }
    }

    internal class Time
    {
        public int Monday { get; set; }
        public int Tuesday { get; set; }
        public int Wednesday { get; set; }
        public int Thursday { get; set; }
        public int Friday { get; set; }
        public int Saturday { get; set; }
        public int Sunday { get; set; }
    }

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication3
{
    class Program
    {
        static void Main(string[] args)
        {            
            int monday = 0;
            int tuesday = 0;
            int wednesday = 0;
            int thursday = 0;
            int friday = 0;
            int saturday = 0;
            int sunday = 0;
            
            DateTime today = DateTime.Now;
            DateTime newday = new DateTime(0001,1,1);
            TimeSpan one_day = new TimeSpan(24,0,0);
            while (newday.ToShortDateString() != today.ToShortDateString())
            {
                newday = newday + one_day;
                if (newday.Day == 13)
                {
                    switch (newday.DayOfWeek.ToString().ToLower())
                    {
                        case "monday":
                            monday++;
                            break;
                        case "tuesday":
                            tuesday++;
                            break;
                        case "wednesday":
                            wednesday++;
                            break;
                        case "thursday":
                            thursday++;
                            break;
                        case "friday":
                            friday++;
                            break;
                        case "saturday":
                            saturday++;
                            break;
                        case "sunday":
                            sunday++;
                            break;
                    }                    
                }                
            }
            Console.WriteLine("Monday: {0} \n Tuesday: {1} \n Wednesday: {2} \n Thursday: {3} \n Friday: {4} \n Saturday: {5} \n Sunday: {6}", 
            monday.ToString(), tuesday.ToString(), wednesday.ToString(),thursday.ToString(),friday.ToString(),saturday.ToString(),sunday.ToString());            
            Console.ReadLine();
        }
    }
}

C#-Code

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;

namespace ConsoleFreitag13
{
    class Program
    {
        const int badDay = 13;
        static void Main(string[] args)
        {
            var days = CountDays(DateTime.MinValue, DateTime.MaxValue).OrderByDescending(a => a.Value);
            var maxLength = days.Max(a => a.Key.ToString().Length);
            foreach (var item in days)
            {
                if (item.Key.DayInMonth == badDay)
                { Console.ForegroundColor = ConsoleColor.Red; }
                else
                { Console.ForegroundColor = ConsoleColor.White; }
                Console.WriteLine($"{item.Key.ToString().PadRight(maxLength)}:  {item.Value}");
            }

            var max = days.Where(b => b.Value == days.Where(c => c.Key.DayInMonth == badDay).Max(a => a.Value) && b.Key.DayInMonth == badDay);
            var min = days.Where(b => b.Value == days.Where(c => c.Key.DayInMonth == badDay).Min(a => a.Value) && b.Key.DayInMonth == badDay);

            Console.WriteLine(new string('-', 40));
            Console.WriteLine($"Die häufigsten Wochentage, die auf einen {badDay}. eines Monats fallen sind:");
            foreach (var item in max)
            {
                Console.WriteLine($"{item.Key.ToString().PadRight(maxLength)}:  {item.Value}");
            }

            Console.WriteLine($"Die seltensten Wochentage, die auf einen {badDay}. eines Monats fallen sind:");
            foreach (var item in min)
            {
                Console.WriteLine($"{item.Key.ToString().PadRight(maxLength)}:  {item.Value}");
            }
            Console.ReadLine();
        }

        static Dictionary<Day, int> CountDays(DateTime from, DateTime to)
        {
            Dictionary<Day, int> result = new Dictionary<Day, int>();

            for (DateTime dt = from; !dt.Date.Equals(to.Date); dt = dt.AddDays(1))
            {
                Day day = new Day() { DayInMonth = dt.Day, DayInWeek = dt.DayOfWeek };
                if (!result.ContainsKey(day))
                {
                    result.Add(day, 0);
                }
                result[day]++;
            }

            return result;
        }
    }

    class Day
    {
        static readonly CultureInfo DE = new CultureInfo("de-DE");

        public int DayInMonth { get; set; }
        public DayOfWeek DayInWeek { get; set; }

        public override bool Equals(object obj)
        {
            Day d = (Day)obj;

            return DayInMonth == d.DayInMonth && DayInWeek == d.DayInWeek;
        }

        public override int GetHashCode()
        {
            int hash = 13;
            hash = (hash * 7) + DayInMonth.GetHashCode();
            hash = (hash * 7) + DayInWeek.GetHashCode();
            return hash;
        }

        public override string ToString()
        {
            return $"{DE.DateTimeFormat.GetDayName(DayInWeek)} der {DayInMonth}.";
        }
    }
}

C#-Code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace der13
{
    class Program
    {
        static int[] weekDayCounter = new int[7];
        static int counter;
        static string weekdayMax;
        static string weekdayMin;
        static int day;
        static int daysave;
        static float i = 1;
        static float numbersave;
        static bool switchYear;

         static void Main(string[] args)
              {
                  Render();
            Console.WriteLine(Convert.ToString(weekDayCounter.Max()));
            if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 0) { weekdayMax = "Monday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 1) { weekdayMax = "Tuesday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 2) { weekdayMax = "Wednesday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 3) { weekdayMax = "Thursday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 4) { weekdayMax = "Friday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 5) { weekdayMax = "Saturday 	"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Max()) == 6) { weekdayMax = "Sunday 	"; }
            Console.WriteLine("The day with the most 13th. is the {0}.", weekdayMax);
            if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 0) { weekdayMin = "Monday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 1) { weekdayMin = "Tuesday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 2) { weekdayMin = "Wednesday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 3) { weekdayMin = "Thursday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 4) { weekdayMin = "Friday"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 5) { weekdayMin = "Saturday 	"; }
            else if (weekDayCounter.ToList().IndexOf(weekDayCounter.Min()) == 6) { weekdayMin = "Sunday 	"; }
            Console.WriteLine("The day with the less 13th. is the {0}.", weekdayMin);
            Console.ReadKey();

        }

         private static void Render()
         {
             for (i = 1; i <= 2018; i++)
             {
                 Schaltjahr();
                 if (i <= 2018 & switchYear == true)
                 {
                     Tag31();
                     Tag29();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag30();
                     Tag31();
                 }
                 if (i <= 2018 & switchYear == false)
                 {
                     Tag31();
                     Tag28();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag31();
                     Tag30();
                     Tag31();
                     Tag30();
                     Tag31();
                 }
             }
         }

         private static void Schaltjahr()
         {
             switchYear = true;
             numbersave = i / 100;
             if (numbersave == (int)numbersave)
             {
                 switchYear = false;
             }

                 numbersave = i / 400;
                 if (numbersave == (int)numbersave)
                 {
                     switchYear = true;
                     numbersave = i / 4;
                     if (numbersave != (int)numbersave & switchYear != false)
                     {
                         switchYear = false;
                     }
                     else
                     {
                         switchYear = true;
                     }
                 }
             
         }
         private static void Tag28()
         {
             while (day <= 7) 
             {
                 Provider();
                 if (counter == 28) { counter = 0; break; }
             }
         }

         private static void Tag29()
         {
             while (day <= 7)
             {
                 Provider();
                 if (counter == 29) { counter = 0; break; }
             }
         }

         private static void Tag31()
         {
             while(day <= 7) 
             {
                 Provider();
                 if (counter == 31) { counter = 0; break; }
             }
         }
         private static void Tag30()
         {
             while (day <= 7) 
             {
                 Provider();
                 if (counter == 30) { counter = 0; break; }
             }
         }

         private static void Tagsaver()
         {
             if (daysave == 1) { weekDayCounter[0]++; }
             if (daysave == 2) { weekDayCounter[1]++; }
             if (daysave == 3) { weekDayCounter[2]++; }
             if (daysave == 4) { weekDayCounter[3]++; }
             if (daysave == 5) { weekDayCounter[4]++; }
             if (daysave == 6) { weekDayCounter[5]++; }
             if (daysave == 7) { weekDayCounter[6]++; }

         }

         private static void Provider()
         {
             counter++;
             day++;
             if (counter == 13) { daysave = day; Tagsaver(); }
             if (day == 7) { day = 0; }
         }
      
    }
}

C#-Code

using System;
using System.Linq;

var sDate = new DateTime(1, 1, 13);
var eDate = new DateTime(2020, 12, 13);
var arrWeekdays = new int[7];

while(sDate <= eDate)
{
    arrWeekdays[(int)sDate.DayOfWeek]++;
    sDate = sDate.AddMonths(1);
}

var lst = Enumerable.Range(0, arrWeekdays.Length).Select(x => (wd: Enum.GetName(typeof(DayOfWeek), x), count: arrWeekdays[x])).ToList();
lst.ForEach(x => Console.WriteLine($"{x.wd}: {x.count}"));
Console.WriteLine($"\nMax: {lst.OrderBy(x => -x.count).FirstOrDefault()}");
Console.WriteLine($"\nMin: {lst.OrderBy(x => x.count).FirstOrDefault()}");