public static function sundays($year = null): array
    {
        // determine the first sunday that is inclusive of all days in the given year
        $year = ($year) ? (int) $year : Date('Y');
        $this_sunday = static::firstSundayOfYear($year);
        if ($this_sunday->format('m-d') != '01-01') {
            $this_sunday->subDays(7);
        }

        // load up the array and return
        $sundays = [];
        while ((int) $this_sunday->format('Y') <= ($year)) {
            $sundays[] = clone $this_sunday;
            $this_sunday = $this_sunday->addDays(7);
        }
        return $sundays;
    }