轉載：
　http://blog.sina.com.cn/u/53c02915010004ao
//快速計算星期幾的公式
#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]);
}