A Function for returning the next NSDate which matches given Day, Hour, Minute, Useful for getting an NSDate to represent Sunday at 4:00 am. This works no matter what the date is now.

getDateFor.swift

import UIKit
import Swift
import XCPlayground

// we want to schedule a timer to fire on Sunday at 4:00am.
// NSTimer takes a fire date.
// NSTimer.init(fireDate date: NSDate, interval seconds: NSTimeInterval, target target: AnyObject, selector aSelector: Selector, userInfo userInfo: AnyObject?, repeats repeats: Bool)
//
// So how do we get the date for the next coming Sunday at 4:00am ?
// Date Components of course!

// Sunday, 4:00 am
let resetDay = 1
let resetHour = 4
let resetMinute = 0


func getDateFor(day: Int, hour: Int, minute: Int) -> NSDate? {
    // Get the Calendar in use
    let cal = NSCalendar.currentCalendar()


    // Get the current day, Hour, Minute
    let todaysComps = cal.components(
        NSCalendarUnit.CalendarUnitWeekday |
        NSCalendarUnit.CalendarUnitHour |
        NSCalendarUnit.CalendarUnitMinute
        , fromDate: NSDate())
    todaysComps.weekday
    todaysComps.hour
    todaysComps.minute

    // Get the relative components, 
    // This is where the real magic happens, How much time between now  and our reset time
    // in days hours minutes
    let resetComps = NSDateComponents()
    resetComps.day    = day - todaysComps.day
    resetComps.hour   = hour - todaysComps.hour
    resetComps.minute = minute - todaysComps.minute

    // Taking the above differences, add them to now
    let date = cal.dateByAddingComponents(resetComps, toDate: NSDate(),
        options: NSCalendarOptions.MatchNextTime)

    return date  //an NSDate? which is the next sunday at 4:00am, useful for settign our timer with
}

let timerDate = getDateFor(resetDay, resetHour, resetMinute)