diff --git a/core/lib/Drupal/Core/Datetime/DateHelper.php b/core/lib/Drupal/Core/Datetime/DateHelper.php
index 6ad8435..4cfc746 100644
--- a/core/lib/Drupal/Core/Datetime/DateHelper.php
+++ b/core/lib/Drupal/Core/Datetime/DateHelper.php
@@ -254,7 +254,9 @@ public static function weekDaysAbbr1($required = FALSE) {
    *   An array of weekdays.
    *
    * @return array
-   *   An array of weekdays reordered to match the first day of the week.
+   *   An array of weekdays reordered to match the first day of the week. The
+   *   keys will remain unchanged. For example, if the first day of the week is
+   *   set to be Monday, the array keys will be [1, 2, 3, 4, 5, 6, 0].
    */
   public static function weekDaysOrdered($weekdays) {
     $first_day = \Drupal::config('system.date')->get('first_day');