如何计算某年某月的第一天是星期几？

下面这段代码我的目的是就算某年某月的第一天是星期几，可是不对，我是按照这种方法做的  ：1900年1月1日星期一 作为标杆
然后算当月第一天距1900.1.1多少天，取余7，不知道哪里有问题，求指教啊
package rili;

import java.util.Scanner;

public class rili {
    boolean isRunNian(int year) {
        if ((year % 400 == 0) || (year % 4 == 0 && year % 100 != 0))
            return true;
        else
            return false;
    }

    int calcDayOfMonth(int year, int month) {
        int days = 0;
        int sum = 0;
        for (int i = 1; i <= month; i++) {
            switch (month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                days = 31;
                break;
            case 4:
            case 6:
            case 9:
            case 11:
                days = 30;
                break;
            case 2:
                if (isRunNian(year))
                    days = 29;
                else
                    days = 28;

            }
            sum += days;
        }
        return sum;
    }

    int calcDays(int year, int month) {
        int sum = 0;
        int days;
        for (int i = 1900; i < year; i++) {
            if (isRunNian(i)) {
                sum += 366;
            } else {
                sum += 365;
            }
        }
        // 对月份中的天数进行处理
        days = calcDayOfMonth(year, month);
        sum += days;
        return sum;

    }

    public static void main(String[] args) {
        // TODO 自动生成的方法存根
        rili r=new rili();
        Scanner s = new Scanner(System.in);
        int year;
        int month;
        year = s.nextInt();
        month = s.nextInt();
        int sum=r.calcDays(year,month-1);
        System.out.println((sum)%7+1);

    }

}