#include <stdio.h>
#include <stdlib.h>

#define MONTHS_IN_YEAR 12
#define DAYS_IN_WEEK 7

int main(void) {
int daysinmonth[MONTHS_IN_YEAR] = { 31, 28, 31, 30, 31, 30, 
                               31, 31, 30, 31, 30, 31 };
char *daynames[DAYS_IN_WEEK] = { "Sunday", "Monday", "Tuesday",
                                 "Wednesday", "Thursday",
                                 "Friday", "Saturday" };

/* read the requested day and month into day and month here ... */

  int day, month, days;
  printf("What day are you interested in?");
  scanf("%d", &day);
  printf("Of which month?");
  scanf("%d", &month);
  if ((day <0) || (day > 31) || (month<0) || (month>12))
    printf("Erroneous input.\n");
  else { 
    days = day-1; /* first account for the days */
    int i;
    for (i=1; i < month; i++) { days = days + daysinmonth[i-1]; }
    days += 4; /* 1st Jan 2015 was Thursday */
    printf("It was a %s\n", daynames[days%DAYS_IN_WEEK]);
  }
  return EXIT_SUCCESS;
}