为什么只有2024年的输出是正确的？

#include <iostream>
#include <iomanip>
using namespace std;
int main(){
    int n=0;
    int y=0;
    int day=0;
    int days=0;
    int a[]={31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    cout<<"请输入年份和月份："<<endl;
    cin>>n>>y;
    if(n<2024){
        cout<<"请输入更大的年份："<<endl;
        return 0;
    }
    if(y<1||y>12){
        cout<<"错误，请重新输入：";
        return 0;
    }
    switch (y) {
        case 12:
            day+=30;
        case 11:
            day+=31;
        case 10:
            day+=30;
        case 9:
            day+=31;
        case 8:
            day+=31;
        case 7:
            day+=30;
        case 6:
            day+=31;
        case 5:
            day+=30;
        case 4:
            day+=31;
        case 3:
            day+=(n % 4 == 0 && n% 100 != 0 || n % 400 == 0) ? 29 : 28;
        case 2:
            day+=31;
    }
    for(int i=2024;i<n;i++){
        if(n % 4 == 0 && n% 100 != 0 || n % 400 == 0){
            days+=366;
        }
        else {
            days+=365;
        }
    }
    int sum=day+days;
    int j=sum%7+1;
    cout <<"---------------------"<< endl;
    cout <<" Su Mo Tu We Th Fr Sa"<< endl;
    cout <<"---------------------"<< endl;
    for(int k=0;k<j;k++){
        cout<<"   ";
    }

    if ((n % 4 == 0 && n% 100 != 0 || n % 400 == 0)&& y == 2){
        a[y-1] += 1;
    }
    int w=7-j;
    for(int i=1;i<=a[y-1];i++){
        cout<<setw(3)<<i;
        if((i-w)%7==0){
            cout<<endl;
        }
    }
    return 0;
}