根据日期计算是星期几

NSDateComponents *_comps = [[NSDateComponents alloc] init];

    [_comps setDay:23];

    [_comps setMonth:3];

    [_comps setYear:2014];

    NSCalendar *gregorian = [[NSCalendar alloc]initWithCalendarIdentifier:NSGregorianCalendar];

    NSDate *_date = [gregorian dateFromComponents:_comps];

    NSDateComponents *weekdayComponents =

    [gregorian components:NSWeekdayCalendarUnit fromDate:_date];

    int _weekday = [weekdayComponents weekday];

    NSLog(@"_weekday::%d",_weekday);