DateTime day=this.monthCalendar1.SelectionStart;
while(day.DayOfWeek!=this.monthCalendar1.FirstDayOfWeek) day=day.AddDays(-1);
   <==此时的day即为所在的周的第一天;
while(day.Month==this.monthCalendar1.SelectionStart.Month) day=day.AddDays(-7);
   <==此时的day即为所在月的第一天;
day=day.AddDays(41);
   <==此时的day即为所在月的最后一天;比如选择今天,则结果依次为:6/22,5/25,7/5