Day name of a given date

#include<stdio.h>

int months[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int leapMonths[] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

int isLeapYear(int year)
{
    if((year%400 == 0) || (year%4 == 0 && year%100 != 0))return 1;
    else return 0;
}

int main()
{
    int d, m, y, days, day1st, i, j;
    while(scanf("%d %d %d", &d, &m, &y) == 3){
        y--;
        day1st = (1 + 5*(y%4) + 4*(y%100) + 6*(y%400))%7;
        days = day1st;

        y++;
        if(isLeapYear(y)){
            for(i = 0; i < m-1; i++){
                days += leapMonths[i];
            }
            days += d;
        }
        else{
            for(i = 0; i < m-1; i++){
                days += months[i];
            }
            days += d;
        }
        day1st = (days-1)%7;
        switch(day1st){
            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");
                break;
        }
    }

    return 0;
}