For Java 15:

private static boolean isLeap(int year){
    if(year < 1918){
        return year % 4 == 0;
    }
    return (year % 400 == 0) || (year%4==0 && year%100!=0);
}

public static String dayOfProgrammer(int year) {
    if(year == 1918){
        return "26.09."+year;
    }
    if(isLeap(year)){
        return "12.09."+year;
    } else {
        return "13.09."+year;
    }
}