We can know all where we made mistakes.

#include <iostream>
using namespace std;
inline bool is_leap_year(int y)
{
    return y%4==0&&(y%100!=0||y%400==0);
}
inline int days_per_year(int y)
{
    if(is_leap_year(y))
        return 366;
    else
        return 365;
}
inline int days_per_month(int m,int y)
{
    const int days[12]={31,28,31,30,31,30,31,31,30,31,30,31};
    if(m!=2)
        return days[m-1];
    else if(is_leap_year(y))
        return 29;
    else
        return 28;
}
int get_day_of_week(int m,int d,int y)
{
    int day=4;
    int i;
    for(i=1600;i<y;i++)
        day+=days_per_year(i);
    for(i=1;i<m;i++)
        day+=days_per_month(i,y);
    day+=d;
    return day%7;
}
void print_calendar(int m,int d,int y)
{
    const char* day_name[7]=
{"mon","tue","wed","thu","fri","sat","sun"};
    int days=days_per_month(m,y);
    int day_begin=1-get_day_of_week(m,1,y);
    for(int i=0;i<7;i++)
    {
        cout<<day_name[i];
        for(int j=day_begin+i;j<=days;j+=7)
            if(j<1||j>days)
                cout<<"     ";
            else if(j==d)
            {
                if(j<10)
                    cout<<" [ "<<j<<"]";
                else
                    cout<<" ["<<j<<"]";
            }
            else
            {
                if(j<10)
                    cout<<"   "<<j<<" ";
                else
                    cout<<"  "<<j<<" ";
            }
        cout<<endl;
    }
}
int main()
{
    int d,m,y;
    cin>>d>>m>>y;
    print_calendar(m,d,y);
    return 0;
}