#include <stdio.h>
#include <stdlib.h>
/* Simple date structure */
typedef struct {
int year;
int month;
int day;
} Date;
/* Check if a year is leap year */
int is_leap(int y) {
if (y % 400 == 0) return 1;
if (y % 100 == 0) return 0;
return (y % 4 == 0);
}
/* Days in each month */
int days_in_month(int y, int m) {
static const int mdays[12] =
{31,28,31,30,31,30,31,31,30,31,30,31};
if (m == 2 && is_leap(y))
return 29;
return mdays[m - 1];
}
/*
Pure arithmetic weekday:
Returns:
0 = Monday
1 = Tuesday
2 = Wednesday
3 = Thursday
4 = Friday
5 = Saturday
6 = Sunday
*/
int weekday_monday0(int y, int m, int d) {
long days = 0;
for (int yr = 1; yr < y; yr++)
days += is_leap(yr) ? 366 : 365;
for (int mo = 1; mo < m; mo++)
days += days_in_month(y, mo);
days += (d - 1);
return (int)(days % 7);
}
/*
// Return the last Sunday of a specific month
*/
Date last_sunday_of_month(int y, unsigned m) {
Date result;
int last = days_in_month(y, m);
/* Walk backward to Sunday */
for (int d = last; d >= 1; d--) {
int w = weekday_monday0(y, m, d);
/* Sunday = 6 */
if (w == 6) {
result.year = y;
result.month = m;
result.day = d;
return result;
}
}
result.year = y;
result.month = m;
result.day = 1;
return result;
}
/*
// Print all last Sundays of each month in a given year
*/
void list_of_last_sundays_of_each_month_in(int year) {
for (unsigned m = 1; m <= 12; ++m) {
Date d = last_sunday_of_month(year, m);
printf("%02d/%02d/%04d\n", d.month, d.day, d.year);
}
}
int main(int argc, char *argv[]) {
int year = 2026;
if (argc > 1) {
year = (int)strtol(argv[1], NULL, 10);
}
list_of_last_sundays_of_each_month_in(year);
return 0;
}
/*
run:
01/25/2026
02/22/2026
03/29/2026
04/26/2026
05/31/2026
06/28/2026
07/26/2026
08/30/2026
09/27/2026
10/25/2026
11/29/2026
12/27/2026
*/
