直接上代碼：

- (void)viewDidLoad {  
    [super viewDidLoad];  
      
    NSLog(@"this day is %@", [self getTheDayOfTheWeekByDateString:@"2015-12-18"]);  
  
    // Do any additional setup after loading the view, typically from a nib  
      
}  
///根據(jù)用戶輸入的時間(dateString)確定當(dāng)天是星期幾,輸入的時間格式 yyyy-MM-dd ,如 2015-12-18  
-(NSString *)getTheDayOfTheWeekByDateString:(NSString *)dateString{  
      
    NSDateFormatter *inputFormatter=[[NSDateFormatter alloc]init];  
      
    [inputFormatter setDateFormat:@"yyyy-MM-dd"];  
      
    NSDate *formatterDate=[inputFormatter dateFromString:dateString];  
      
    NSDateFormatter *outputFormatter=[[NSDateFormatter alloc]init];  
      
    [outputFormatter setDateFormat:@"EEEE-MMMM-d"];  
      
    NSString *outputDateStr=[outputFormatter stringFromDate:formatterDate];  
      
    NSArray *weekArray=[outputDateStr componentsSeparatedByString:@"-"];  
      
    return [weekArray objectAtIndex:0];  
}