You can use this table date_formats for converting your date to different formats. My shortest code:
func getDayOfWeek(today: String) -> Int{
let formatter:NSDateFormatter = NSDateFormatter()
formatter.dateFormat = "yyyy-MM-dd"
let todayDate = formatter.dateFromString(today)
formatter.dateFormat = "e" // "eeee" -> Friday
let weekDay = formatter.stringFromDate(todayDate!)
return Int(weekDay)!
}
getDayOfWeek("2015-12-18") // 6