Find the day of a given date -- C program

dayFromDate.c

#include <stdio.h>

int main () {
	int d=27, m=6, y=2015, weekday ;
	printf("Enter date, month and year: ");
	scanf("%d%d%d",&d,&m,&y);
	weekday  = (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7  ;
	printf("The day of the week is : %d",weekday);
	switch(weekday) {
		case 0:
			printf(" Sunday.\n");
			break;
		case 1:
                        printf(" Monday.\n");
			break;
		case 2:
                        printf(" Tuesday.\n");
			break;
		case 3:
                        printf(" Wednesday.\n");
			break;
		case 4:
                        printf(" Thursday.\n");
			break;
		case 5:
                        printf(" Friday.\n");
			break;
		case 6:
                        printf(" Saturday.\n");
	}
	return 0;
}

I was unable to understand the logic for the weekday can any one help me please.

I too face the same problem.
Can you explain it clearly?

For what I determined this could be a variant or simplification of Zeller's congruence but I'm either too dumb or I don't have time at the moment to understand the modifications. It works tho, so that's good.
Also, the line weekday = (d+=m<3?y--:y-2,23*m/9+d+4+y/4-y/100+y/400)%7 ; could be expanded as

d+=m<3?y--:y-2;
weekday  = Math.floor((23*m/9+d+4+y/4-y/100+y/400)%7 );

to port this code to javascript.

@tarun6196 @buela25

thank you, works fine