NSCalendar *gregorian = [[[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar] autorelease];
NSDateComponents *comps = [gregorian components:NSWeekdayCalendarUnit fromDate:[NSDate date]];
int weekday = [comps weekday];
NSLog(@"the wee %d",weekday);
NSDate *lastSunday = [[NSDate date] addTimeInterval:-3600*24*(weekday-1)];
NSLog(@"nsdar %@",newDate);
это дает вам последнее воскресенье в формате NSDate
. Надеюсь, это поможет.