Language: C++

#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cstdlib>
#include <cmath>
#include <numeric>

using namespace std;

#define rep(i, n) for(int i = 0; i < n; i++)
#define repi(i, start, end) for(int i = start; i < end; i++)
#define all(x) (x).begin(), (x).end()
#define debug(x) cout << (x) << endl;
typedef long long ll;
const int MAX = 999999999;

int main(){
   int d;
   cin >> d;

    if(d == 25) cout << "Christmas" << endl;
    else if(d == 24) cout << "Christmas Eve" << endl;
    else if(d == 23) cout << "Christmas Eve Eve" << endl;
    else if(d == 22) cout << "Christmas Eve Eve EVe" << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:20:4: error: 'cin' was not declared in this scope
   20 |    cin >> d;
      |    ^~~
a.cc:8:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
    7 | #include <numeric>
  +++ |+#include <iostream>
    8 | 
a.cc:22:17: error: 'cout' was not declared in this scope
   22 |     if(d == 25) cout << "Christmas" << endl;
      |                 ^~~~
a.cc:22:17: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:22:40: error: 'endl' was not declared in this scope
   22 |     if(d == 25) cout << "Christmas" << endl;
      |                                        ^~~~
a.cc:8:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
    7 | #include <numeric>
  +++ |+#include <ostream>
    8 | 
a.cc:23:22: error: 'cout' was not declared in this scope
   23 |     else if(d == 24) cout << "Christmas Eve" << endl;
      |                      ^~~~
a.cc:23:22: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:23:49: error: 'endl' was not declared in this scope
   23 |     else if(d == 24) cout << "Christmas Eve" << endl;
      |                                                 ^~~~
a.cc:23:49: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:24:22: error: 'cout' was not declared in this scope
   24 |     else if(d == 23) cout << "Christmas Eve Eve" << endl;
      |                      ^~~~
a.cc:24:22: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:24:53: error: 'endl' was not declared in this scope
   24 |     else if(d == 23) cout << "Christmas Eve Eve" << endl;
      |                                                     ^~~~
a.cc:24:53: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:25:22: error: 'cout' was not declared in this scope
   25 |     else if(d == 22) cout << "Christmas Eve Eve EVe" << endl;
      |                      ^~~~
a.cc:25:22: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:25:57: error: 'endl' was not declared in this scope
   25 |     else if(d == 22) cout << "Christmas Eve Eve EVe" << endl;
      |                                                         ^~~~
a.cc:25:57: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'