hdu 2133 What day is it

#include <stdio.h>

int b[20] = {0,31,28,31,30,31,30,31,31,30,31,30,31,0};
int Y = 1, M = 1, D = 1;
int leap(int y)
{
if(y%400 == 0 || (y%4 == 0 && y%100!=0))
return 1;
return 0;
}

int main()
{
int y,m,d,i;
while(~scanf("%d%d%d",&y,&m,&d))
{
if(leap(y))
b[2] = 29;
else
b[2] = 28;
if(d > b[m] || d < 1 || m < 1 || m > 12 || y <1 || y>10000)
{
printf("illegal\n");
continue;
}
int sum = 0;
sum+=d;
for(i = 0; i<m; i++)
sum+=b[i];
sum%=7;
for(i = 1; i<y; i++)
{
sum+=365;
if(leap(i))
sum++;
sum%=7;
}
int  dx = sum%7;
if(dx==0)
printf("Sunday\n");
else if(dx == 1)
printf("Monday\n");
else if(dx == 2)
printf("Tuesday\n");
else if(dx == 3)
printf("Wednesday\n");
else if(dx == 4)
printf("Thursday\n");
else if(dx == 5)
printf("Friday\n");
else if(dx == 6)
printf("Saturday\n");
}
return 0;
}