实现相关日历的计算，当用户输入一个月份，程序输出这个月的月历，每星期一行，星期天开始，星期六结束。当用户输入一个日期，程序显示这个日期当天是星期几。当用户输入两个日期，程序计算这两个日期相差的天数。

-To achieve the calculation of the relevant calendar, when the user enters a month, the program output this month' s calendar, every Monday line, beginning Sunday, ending Saturday. When users enter a date, the program shows this date is the day of the week. When the user enters two dates, the program calculated the number of days difference between two dates.