"Create change window" allows custom rules for scheduling recurring change windows as "day X on every month". This won't work well as day X in some months could be on the weekend. Administrators want more flexible rules such as "1st/2nd/3rd Thursday".     

The suggestion is to allow cron expression for the custom recurring rule to give more flexibility as already being implemented in the other component of Jira. e.g.  https://support.atlassian.com/jira-software-cloud/docs/manage-filters/