printf("%s\n%*s", "Mo Di Mi Do Fr Sa So", first_day-1, "   ");
for(int i = 1;i<=num_days;i++)
{
   printf("%3d", i);
   if ((i+first_day-1) % 7 == 0) printf("\n")
}

Но цель задания - не календарь, а тренировка техники программирования

const int cell_len = 3;
const int max_len = 37;
char * calender  = null;
calender = malloc(max_len*cell_len*sizeof(char));
memset (calender, ' ', max_len*cell_len);
char days[] = "  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31";
memcpy(calender+(first_day-1)*cell_len, days, num_days * cell_len);
calender[7*cell_len] = '\n';
calender[14*cell_len] = '\n';
calender[21*cell_len] = '\n';
calender[28*cell_len] = '\n';
calender[first_day*cell_len+ num_days * cell_len] = '\0'; 
printf("Mo Di Mi Do Fr Sa So\n%s", calender);
free (calender);

?