public static function factory($days_str) {
$limits = explode('-', $days_str);
$days = array();
$init = $limits[0];
$final = $limits[1];
$current = new day($init);
$days[] = $current;
while($current->abbrev() != $final) {
$current = $current->next();
$days[] = $current;
}
return $days;
}
