获取周一还是周日作为首日
first = DateTime.MinValue;
last = DateTime.MinValue;
DateTime firstDay = new DateTime(year, 1, 1);
int firstDayOfWeek = (int)new DateTime(year, 1, 1).DayOfWeek;
first = firstDay.AddDays((weeks - 1) * 7 - firstDayOfWeek + 1); //星期一
last = first.AddDays(6);
first = firstDay.AddDays((weeks - 1) * 7 - firstDayOfWeek); //星期天
last = first.AddDays(6);