/**
  * Return an i18n'ified associative array of all possible subscription periods.
  *
  * @since 1.5
  */
 public static function get_billing_period_ranges($billing_period = '')
 {
     global $wp_locale;
     if (empty(self::$billing_period_ranges)) {
         foreach (array('week', 'month', 'year') as $key) {
             self::$billing_period_ranges[$key][0] = __('Do not synchronise', 'woocommerce-subscriptions');
         }
         // Week
         $weekdays = array_merge($wp_locale->weekday, array($wp_locale->weekday[0]));
         unset($weekdays[0]);
         foreach ($weekdays as $i => $weekly_billing_period) {
             self::$billing_period_ranges['week'][$i] = sprintf(__('%s each week', 'woocommerce-subscriptions'), $weekly_billing_period);
         }
         // Month
         foreach (range(1, 27) as $i) {
             self::$billing_period_ranges['month'][$i] = sprintf(__('%s day of the month', 'woocommerce-subscriptions'), WC_Subscriptions::append_numeral_suffix($i));
         }
         self::$billing_period_ranges['month'][28] = __('Last day of the month', 'woocommerce-subscriptions');
         self::$billing_period_ranges = apply_filters('woocommerce_subscription_billing_period_ranges', self::$billing_period_ranges);
     }
     if (empty($billing_period)) {
         return self::$billing_period_ranges;
     } elseif (isset(self::$billing_period_ranges[$billing_period])) {
         return self::$billing_period_ranges[$billing_period];
     } else {
         return array();
     }
 }