A simple Swift function to find the day of the week of a given day.

DayFinder.swift

import Foundation

enum DateError: Error {
    case IllegalFormat
    case DayOutOfRange
}

/**
Gets the day of the week of a given date on the Gregorian calendar.

- parameter date: A date containing a month, date, and 4-digit year.

- throws: Errors of type `DateError`:
    - `DateError`.`IllegalFormat` — the date string provided is not in one of the supported formats
    - `DateError`.`DayOutOfRange` — the `Weekday` component of the given day is not between 1 and 7

- returns: A string containing the day of the week on which the specified date occurred.
*/
func getDay(date: String) throws -> String {
    
    let df = DateFormatter()
    df.timeStyle = .none
    df.dateFormat = "MM/dd/yyyy"
    
    var finalDate: Date!
    
    let mmddyyyy = df.date(from: date)
    if mmddyyyy == nil {
        let df2 = DateFormatter()
        df2.timeStyle = .none
        df2.dateFormat = "MM dd, yyyy"
        finalDate = df2.date(from: date)
    } else {
        finalDate = mmddyyyy
    }
    
    if let date = finalDate {
        
        let calendar = NSCalendar(calendarIdentifier: NSCalendar.Identifier.gregorian)
        let day = calendar!.component(.weekday, from: date as Date)
        
        switch day {
        case 1:
            return "Sunday"
        case 2:
            return "Monday"
        case 3:
            return "Tuesday"
        case 4:
            return "Wednesday"
        case 5:
            return "Thursday"
        case 6:
            return "Friday"
        case 7:
            return "Saturday"
        default:
            throw DateError.DayOutOfRange
        }
    } else {
        throw DateError.IllegalFormat
    }
}