当月第一天星期几，跟时区有关系。

设置里面设置时区会有错误-----需要设置setFirstWeekday：
NSCalendar *calendar = [NSCalendar currentCalendar];
[calendar setFirstWeekday:1];//1.Sun. 2.Mon. 3.Thes. 4.Wed. 5.Thur. 6.Fri. 7.Sat.

• (NSInteger)firstWeekDayInMonth{
  NSCalendar *calendar = [NSCalendar currentCalendar];
  [calendar setFirstWeekday:1];//1.Sun. 2.Mon. 3.Thes. 4.Wed. 5.Thur. 6.Fri. 7.Sat.
  NSDateComponents *comp = [calendar components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay) fromDate:self];
  [comp setDay:1];
  NSDate *firstDayOfMonthDate = [calendar dateFromComponents:comp];

  NSUInteger firstWeekday = [calendar ordinalityOfUnit:NSCalendarUnitWeekday inUnit:NSCalendarUnitWeekOfMonth forDate:firstDayOfMonthDate] - 1;
  return firstWeekday;
  }