已知公元一年一月一日是星期一，求任意一天是星期几?
#include<stdio.h>
void main()
{
 int year,month,day,sum;
 printf("Please input years march:");
 scanf("%d %d %d",&year,&month,&day);
 switch(month)
 {
  case 1:sum=0;break;
  case 2:sum=31;break;
  case 3:sum=59;break;
  case 4:sum=90;break;
  case 5:sum=120;break;
  case 6:sum=151;break;
  case 7:sum=181;break;
  case 8:sum=212;break;
  case 9:sum=243;break;
  case 10:sum=273;break;
  case 11:sum=304;break;
  case 12:sum=334;break;
 }
 sum=sum+day+(year-1)*365;
 if(year%400==0||(year%4==0&&year%100!=0))
 sum=sum+1;
 if(sum%7==0)
 printf("Today is Sunday.\n");
 else printf("Today is%d\n",sum%7);
}

应该先由year判断经过了几个闰年，再结合month和day计算sum