求一函数，输入日期，输出星期几

输入日期...要写不少
#include "stdio.h"
main()
{
int day_num;
printf("enter a value between 1 and 7 : ");
scanf("%d",&day_num);

switch (day_num){
case 1: printf("The day is monday\n");
break;
case 2:printf("the day is tuesday\n");
break;
case 3:printf("the day is wednesday\n");
break;
case 4:printf("the day is thursday\n");
break;
case 5:printf("the day is friday\n");
break;
case 6:printf("the day is saturday\n");
break;
case 7:printf("the day is sunday\n");
break;
default:printf("you input number is not valid");
break;
}
}

/*
1582/10/4之后时
W=[C/4]-2C+y+[y/4]+[13*(M+1)/5]+d-1
1582/10/4之前时
w=y+[y/4]+[c/4]-2c+[13(m+1)/5]+d+2

w：星期；
c：