This piece of code should get what you want. It simply calculates how many days are from monday and append it from current's date.
NSDate *now = [NSDate date];
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [calendar components:NSYearCalendarUnit | NSMonthCalendarUnit | NSWeekCalendarUnit | NSWeekdayCalendarUnit fromDate:now];
NSUInteger weekdayToday = [components weekday];
NSInteger daysToMonday = (9 - weekdayToday) % 7;
NSDate *nextMonday = [now dateByAddingTimeInterval:60*60*24*daysToMonday];
Untested, but should work, and without worrying about changing first dates of calendar.
And it can even be easily addapted to every another day of the week, just change the 9inside (9 - weekdayToday) % 7; by 7 + weekDayYouWant, remembering that sunday = 1, monday = 2...