转载：
　http://blog.sina.com.cn/u/53c02915010004ao
//快速计算星期几的公式
#include<stdio.h>
char *name[] = { "monday",
                 "tuesday",
                 "wednesday",
                "thursday",
                "friday",
                "saturday",
                "sunday"
               };
void main(){
  int D,M,Y,A;
  scanf("%d%d%d",&Y,&M,&D);
  if ((M == 1) || (M == 2)){
    M += 12;
    Y--;
  }
  A = (D + 2*M + 3*(M+1)/5 + Y + Y/4 - Y/100 + Y/400) % 7;
  printf("%s\n",name[A]);
}