func dayType(for day: String) -> String {
  switch day {
    case: "Saturday", "Sunday": return "Weekend"
    case: "Monday", "Tuesday", "Wednessday", "Thursday", "Friday": return "Weekday"
    default: return "This is not a valid date"
  }
}

let result1 = dayType(for: "Sunday") //will return "Weekend"
let result2 = dayType(for: "Thursday") //will return "Weekday"

let result3 = dayType(for: "fridayys") //will return "This is not a valid date"