/*

calculates the name of the weekday for the 1st day of a given month & year

Michael Sanders 2024
https://busybox.neocities.org/notes/get-first-day-of-month.txt

Further reading...
http://datagenetics.com/blog/november12019/index.html
https://www.themathdoctors.org/zellers-rule-what-day-of-the-week-is-it/
https://en.wikipedia.org/wiki/Zeller%27s_congruence

*/

#include <stdio.h>

const char *weekdays[7] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };

int getFirstDayOfMonth(int year, int month) {

    /*

    Zeller's Congruence Algorithm calculates the name of the
    weekday for the 1st day of the month (Sun, Mon, etc)
    note: months are indexed 1 to 12

    */

    if (month < 3) {
        month += 12;
        year -= 1;
    }

    int K = year % 100;
    int J = year / 100;
    int d = 1;  // 1st day of the month
    int f = d + 13 * (month + 1) / 5 + K + K / 4 + J / 4 + 5 * J;

    return (f % 7) - 1;

}

int main(void) {

    const char *FirstDayOfAugust = weekdays[getFirstDayOfMonth(2024, 8)];
    printf("First day of August 2024: %s\n", FirstDayOfAugust);
    return 0;

}

// eof