How to get all days in a month that are a specific weekday?

A DateComponents has a weekday property, representing the day of the week. The weekdays are (in Foundation's Gregorian calendar) numbered 1 for Sunday, 2 for Monday, …, and 7 for Saturday.

A DateComponents also has a weekdayOrdinal property, representing “the position of the weekday within the next larger calendar unit, such as the month. For example, 2 is the weekday ordinal unit for the second Friday of the month.”

So let's initialize a DateComponents for some Saturday in June 2017. It's generally a good idea to specify a time of noon if you don't care about the time, because midnight (the default time of day) can cause problems in some time zones on some days.

var components = DateComponents(era: 1, year: 2017, month: 06, hour: 12, weekday: 7)

And let's make a calendar.

var calendar = Calendar.autoupdatingCurrent

Now we can loop over all the possible weekday ordinals. For each, we'll ask the calendar to generate a date. Then we ask the calendar to convert the date back to year, month, and day components.

In the Gregorian calendar, some months have 5 Saturdays, but most have 4. So when we ask for the 5th Saturday, we'll probably get a date in the following month. When that happens, we want to suppress that date.

for i in 1 ... 5 {
    components.weekdayOrdinal = i
    let date = calendar.date(from: components)!
    let ymd = calendar.dateComponents([.year, .month, .day], from: date)
    guard ymd.month == components.month else { break }
    print("\(ymd.year!)-\(ymd.month!)-\(ymd.day!)")
}

Output:

2017-6-3
2017-6-10
2017-6-17
2017-6-24

Objective-C version:

NSDateComponents *components = [NSDateComponents new];
components.era = 1;
components.year = 2017;
components.month = 6;
components.hour = 12;
components.weekday = 7;
NSCalendar *calendar = NSCalendar.autoupdatingCurrentCalendar;

for (NSInteger i = 1; i <= 5; ++i) {
    components.weekdayOrdinal = i;
    NSDate *date = [calendar dateFromComponents:components];
    NSDateComponents *ymd = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay fromDate:date];
    if (ymd.month != components.month) { break; }
    NSLog(@"%ld-%ld-%ld", (long)ymd.year, (long)ymd.month, (long)ymd.day);
}