// A month has five full weekends when it has 31 days, and the 1st day of the month is a Friday.
#include <stdio.h>
#include <stdbool.h>
// ------------------------------------------------------------
// Month names as a single reusable constant
// ------------------------------------------------------------
const char *monthNames[12] = {
"January", "February", "March", "April", "May", "June",
"July", "August", "September", "October", "November", "December"
};
// ------------------------------------------------------------
// Zeller's congruence (returns 0=Saturday, 1=Sunday, ..., 6=Friday)
// ------------------------------------------------------------
int weekday_of_date(int year, int month, int day) {
if (month < 3) {
month += 12;
year -= 1;
}
int K = year % 100;
int J = year / 100;
// Zeller’s congruence - formula for computing the day of the week
// for any Gregorian date.
int h = (day + (13 * (month + 1)) / 5 + K + K/4 + J/4 + 5*J) % 7;
return h; // 0=Saturday, 6=Friday
}
// ------------------------------------------------------------
// Function: returns true if a month has 5 full weekends
// ------------------------------------------------------------
bool hasFiveFullWeekends(int year, unsigned m) {
int daysInMonth[12] = {
31,28,31,30,31,30,31,31,30,31,30,31
};
// Leap year adjustment
bool leap = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
if (leap)
daysInMonth[1] = 29;
int days = daysInMonth[m - 1];
// First day of the month
int wd = weekday_of_date(year, m, 1);
// Zeller: Friday = 6
return (days == 31 && wd == 6);
}
// ------------------------------------------------------------
// Main program
// ------------------------------------------------------------
int main() {
const int y_value = 2026;
for (int m = 1; m <= 12; m++) {
if (hasFiveFullWeekends(y_value, (unsigned)m)) {
printf("%s %d has five full weekends.\n",
monthNames[m - 1], y_value);
}
}
return 0;
}
/*
run:
May 2026 has five full weekends.
*/
