hdu 2133 What day is it

#include<iostream>
#include<cstdio>
using namespace std;
int da[2][13]={
{0,31,28,31,30,31,30,31,31,30,31,30,31},
{0,31,29,31,30,31,30,31,31,30,31,30,31},
};
char w[8][10]={"Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"};
int isLeap(int y)
{
int  ret = 0;
if(y%4==0&&y%100!=0||y%400==0) ret = 1;
return ret;
}
int main()
{
int y,m,d,sum;
while(cin>>y>>m>>d)
{
sum = 0;
if(m>12||m<=0||d>da[isLeap(y)][m]||d<=0)
{
cout<<"illegal"<<endl;
continue;
}
for(int i=1;i<y;i++)
{
if(isLeap(i)) sum+=366;
else sum+=365;
}
for(int i=1;i<m;i++) sum+=da[isLeap(y)][i];
sum+=d;
cout<<w[sum%7]<<endl;
}
}