private static function init()
    {
        $day = Calendar::getFirstWeekday();
        $day = $day == 0 ? 7 : $day;
        self::$weekStartsAt = $day;

        $days = self::$days;
        while (array_keys($days)[0] != $day) {
            $key = array_keys($days)[0];
            $value = $days[$key];
            unset($days[$key]);
            $days[$key] = $value;
        }

        self::$init = true;
    }