Zeller

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static void usage(const char *program_name) {
  fprintf(stderr, "Usage: %s yyyy [m]m [d]d\n", program_name);
}
int main(const int argc, const char **argv) {
  int i, j, len, y, m, d, leap = 0, day_of_week;
  const int days[][13] = {
      { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 },
      { 0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 } };
  const char *name[] = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" };
  if (argc != 4) goto error;
  for (i = 1; i < 4; i++) {
    len = strlen(argv[i]);
    if (i == 1 && len < 4) goto error;
    if ((i == 2 || i == 3) && len > 2) goto error;
    for (j = 0; j < len; j++)
      if (!isdigit(argv[i][j])) goto error;
  }
  y = atoi(argv[1]);
  m = atoi(argv[2]);
  d = atoi(argv[3]);
  if (y < 1582) goto error;
  leap = (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
  if (m < 1 || 12 < m) goto error;
  if (d < 1 || days[leap][m] < d) goto error;
  if (m == 1 || m == 2) {
    y--;
    m += 12;
  }
  day_of_week = (y + y / 4 - y / 100 + y / 400 + (13 * m + 8) / 5 + d) % 7;
  printf("%s\n", name[day_of_week]);
  return 0;
  error: usage(argv[0]);
  return 1;
}