CalendR\Period\Factory::findFirstDayOfWeek PHP Метод

    public function findFirstDayOfWeek($dateTime)
    {
        $day = clone $dateTime;
        $delta = ((int) $day->format('w') - $this->getFirstWeekday() + 7) % 7;
        $day->sub(new \DateInterval(sprintf('P%sD', $delta)));
        return $day;
    }