WordPress: Theme Switcher by a period of the year

add_filter('template','date_switch_theme');
function date_switch_theme($theme) {
       $month = date('n');
       if ($month >= 1 && $month <= 4) return 'themename-1';
       if ($month >= 5 && $month <= 8) return 'themename-2';
       if ($month >= 9 && $month <= 12) return 'themename-3';
       return $theme; // shouldn't happen
}

//este código é apenas um teste para trocar o tema a cada 4 meses.