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;
}

Go: package main

import (
    "fmt"
    "time"
)

type holiday struct {
    time.Time
}

func easter(y int) holiday {
    c := y / 100
    n := mod(y, 19)
    i := mod(c-c/4-(c-(c-17)/25)/3+19*n+15, 30)
    i -= (i / 28) * (1 - (i/28)*(29/(i+1))*((21-n)/11))
    l := i - mod(y+y/4+i+2-c+c/4, 7)
    m := 3 + (l+40)/44
    d := l + 28 - 31*(m/4)
    return holiday{time.Date(y, time.Month(m), d, 0, 0, 0, 0, time.UTC)}
}

func mod(a, n int) int {
    r := a % n
    if r < 0 {
        return r + n
    }
    return r
}

func (h holiday) addDays(d int) holiday {
    return holiday{h.Add(time.Duration(d) * 24 * time.Hour)}
}

type easterRelated struct {
    easter, ascension, pentecost, trinity, corpusChristi holiday
}

func newEasterRelated(y int) (er easterRelated) {
    er.easter = easter(y)
    er.ascension = er.easter.addDays(39)
    er.pentecost = er.ascension.addDays(10)
    er.trinity = er.pentecost.addDays(7)
    er.corpusChristi = er.trinity.addDays(4)
    return
}

func (er easterRelated) print() {
    const wdmdm = ": Mon _2 Jan"
    fmt.Printf("%4d %s, %s, %s, %s, %s\n",
        er.easter.Year(),
        er.easter.Format("Easter"+wdmdm),
        er.ascension.Format("Ascension"+wdmdm),
        er.pentecost.Format("Pentecost"+wdmdm),
        er.trinity.Format("Trinity"+wdmdm),
        er.corpusChristi.Format("Corpus"+wdmdm))
}

func main() {
    fmt.Println("Christian holidays, related to Easter, " +
        "for each centennial from 400 to 2100 CE:")
    for y := 400; y <= 2100; y += 100 {
        newEasterRelated(y).print()
    }
    fmt.Println("\nChristian holidays, related to Easter, " +
        "for years from 2010 to 2020 CE:")
    for y := 2010; y <= 2020; y++ {
        newEasterRelated(y).print()
    }
}