Task: Holidays related to Easter
C: #include <stdio.h>

typedef int year_t, month_t, week_t, day_t;

typedef struct{
  year_t year; 
  month_t month;  day_t month_day; day_t week_day; } date_t;

const char *mon_fmt[] =      {0, "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                            "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"};
const char *week_day_fmt[] = {0, "Sat", "Sun", "Mon", "Tue", "Wed", "Thu", "Fri"};

day_t month_days(year_t year, month_t month)
  { day_t days[]={0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    return (month==2) ? 28 + (( year % 4 == 0 && year % 100 != 0 ) || year % 400 == 0) : days[month];
  }

day_t year_days(year_t year) 
  { return (month_days(year, 2) == 28) ? 365 : 366; }

month_t year_months = 12;
week_t week_days = 7; 

date_t plusab(date_t *date, day_t days)
    { 
    
       while(days < 0){
         date->year -= 1;
         days += year_days(date->year);
       };
       while(days > year_days(date->year)){
         days -= year_days(date->year);
         date->year += 1;
       };
       date->month_day += days;
    
       while(date->month_day > month_days(date->year, date->month)){
          date->month_day -= month_days(date->year, date->month);
          date->month += 1;
          if(date->month > year_months){
             date->month -= year_months;
             date->year += 1;
          }
       }
       date->week_day = week_day(*date);
       return *date;
    }

date_t easter (year_t year)
   {
      
      date_t date; date.year = year;
      int c = year / 100, n = year % 19; 
      int i = (c - c / 4 - (c - (c - 17) / 25) / 3 + 19 * n + 15) % 30;
      i -= (i / 28) * (1 - (i / 28) * (29 / (i + 1)) * ((21 - n) / 11));
      int l = i - (year + year / 4 + i + 2 - c + c / 4) % 7;
      date.month = 3 + (l + 40) / 44;
      date.month_day = l + 28 - 31 * (date.month / 4);
      date.week_day = week_day(date);
      return date;
   }

day_t week_day (date_t date)
   
   {
      int year = date.year, month = date.month, month_day = date.month_day, c;
      if(month <= 2){month += 12; year -= 1;}
      c = year / 100;
      year %= 100;
      return 1 + ((month_day + ((month + 1) * 26) / 10
         + year + year / 4 + c / 4 - 2 * c) % 7 + 7) % 7;
   }

#define wdmdm_fmt "%s %2d %s"

typedef struct{date_t easter, ascension, pentecost, trinity, corpus_christi;}easter_related_t; 

easter_related_t easter_related_init (year_t year)
{
   date_t date;
   easter_related_t holidays;

   holidays.easter = date = easter(year);

   holidays.ascension = plusab(&date, 39);

   holidays.pentecost = plusab(&date, 10);

   holidays.trinity = plusab(&date, 7);

   holidays.corpus_christi = plusab(&date, 4);
   return holidays;
}


#define easter_related_fmt "%4d Easter: "wdmdm_fmt", Ascension: "wdmdm_fmt\
    ", Pentecost: "wdmdm_fmt", Trinity: "wdmdm_fmt", Corpus: "wdmdm_fmt"\n"

void easter_related_print(year_t year)
{
  easter_related_t holidays = easter_related_init(year);
#define wdmdm(date) week_day_fmt[date.week_day], date.month_day, mon_fmt[date.month]
  printf(easter_related_fmt, year,
    wdmdm(holidays.easter),  wdmdm(holidays.ascension), wdmdm(holidays.pentecost),
    wdmdm(holidays.trinity), wdmdm(holidays.corpus_christi));
}

int main(){
  year_t year;

  printf ("Christian holidays, related to Easter, for each centennial from 400 to 2100 CE:\n");
  for(year=400; year<=2100; year+=100){ easter_related_print(year); }

  printf ("\nChristian holidays, related to Easter, for years from 2010 to 2020 CE:\n");
  for(year=2010; year<=2020; year++){ easter_related_print(year); }
  return 0;
}

C#: using System;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;

internal class Program
{
    private static readonly OrderedDictionary _holidayOffsets = new OrderedDictionary
                                                                    {
                                                                        {"Easter", 0},
                                                                        {"Ascension", 39},
                                                                        {"Pentecost", 49},
                                                                        {"Trinity", 56},
                                                                        {"Corpus", 60},
                                                                    };

    static void Main(string[] args)
    {
        Console.WriteLine("Christian holidays, related to Easter, for each centennial from 400 to 2100 CE:");
        for (int year = 400; year <= 2100; year += 100)
            OutputHolidays(year);

        Console.WriteLine();
        Console.WriteLine("Christian holidays, related to Easter, for years from 2010 to 2020 CE:");
        for (int year = 2010; year <= 2020; year += 1)
            OutputHolidays(year);
    }

    static void OutputHolidays(int year)
    {
        var easter = CalculateEaster(year);
        var holidays = from kp in _holidayOffsets.OfType<DictionaryEntry>()
                       let holiday = easter.AddDays(Convert.ToInt32(kp.Value))
                       select kp.Key + ": " + string.Format("{0,2:ddd} {0,2:%d} {0:MMM}", holiday);
        Console.WriteLine("{0,4} {1}", year, string.Join(", ", holidays.ToArray()));
    }

    static DateTime CalculateEaster(int year)
    {
        var a = year % 19;
        var b = year / 100;
        var c = year %100;
        var d = b / 4;
        var e = b % 4;
        var f = (b + 8) / 25;
        var g = (b - f + 1) / 3;
        var h = (19 * a + b - d - g + 15) % 30;
        var i = c / 4;
        var k = c % 4;
        var l = (32 + 2 * e + 2 * i - h - k) % 7;
        var m = (a + 11 * h + 22 * l) / 451;
        var numerator = h + l - 7 * m + 114;
        var month = numerator / 31;
        var day = (numerator % 31) + 1;
        return new DateTime(year, month, day);
    }
}