Per this announcement there are now centralized tables of holidays: https://microsoftteams.uservoice.com/forums/555103-public/suggestions/16934143-rename-of-team-channel-doesn-t-rename-associated-s

The problem is the UI for the Auto Attendant only allows you to pick one holiday when setting up a holiday call flow. Even though you can maintain a centralized list of holidays, this means you still have to edit your Auto Attendant holiday call flow and setup a unique call flow for each holiday independently.

You should also have the same options to select for a holiday call flow as you do for the after hrs call flow.