Based on Zeller's formula, only January and February have different results than expected.

#include<stdio.h>
int main (void)
{
    int year, m, q, k, j, h;

    printf("year:");
    scanf("%d", & year);
    printf("Month:");
    scanf("%d", & m);
    printf("Day:");
    scanf("%d", & q);
    k = year%100;
    j = year/100;
    h = (q+(m+1)*26)/10+k+k/4+j/4+5*j)%7;
   /* On or before October 14, 1582, */
    if(year<1582||year==1582&&(m<10||m==10&&q<=14)){
    printf("Not supported before October 14, 1582);
    }
    else if(h==1){
    printf("%4d year %2d month %2d day is Sunday\n", year, m, q);
    }
    else if(h==2){
    printf("%4d year %2d month %2d day is Monday\n", year, m, q);
    } 
    else if(h==3){
    printf("%4d year %2d month %2d day is Tuesday\n", year, m, q);
    }
    else if(h==4){
    printf("%4d year %2d month %2d day is Wednesday\n", year, m, q);
    }
    else if(h==5){
    printf("%4d year %2d month %2d day is Thursday\n", year, m, q);
    }
    else if(h==6){
    printf("%4d year %2d month %2d day is Friday\n", year, m, q);
    } 
    else if(h==0){
    printf("%4d year %2d month %2d day is Saturday\n", year, m, q);
    }
    return 0;
}

$./a.out
Year: 2020
Month: 2
Day: 7
February 7th, 2020 is Wednesday.

$ ./a.out
Year: 2020
Month: 1
Day: 31
January 31st, 2020 is Thursday.

$ ./a.out
Year: 2020
Month: 3
Day: 7
March 7th, 2020 is Saturday.

$ ./a.out
Year: 2020
Month: 4
Day: 6
April 6th, 2020 is Monday.