如何计算某一天是星期几?

 1 #include <stdio.h>  
 2 
 3 int main()
 4 {
 5     int year, month, day, c, y, w;
 6     char b[7][10] = { "sunday", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday" };
 7     scanf("%d%d%d", &year, &month, &day);
 8     if (month < 3)
 9     {
10         year--;
11         month += 12;
12     }
13     c = year / 100;
14     y = year % 100;
15     w = (c / 4) - 2 * c + (y + y / 4) + (13*(month + 1) / 5) + day - 1;
16     w = (w % 7 + 7) % 7;
17     puts(b[w]);
18     return 0;
19 }