Hi,
This is not really what custom fields are for.
I would suggest that instead of using a custom field you can create your own taxonomy (also possible with Custom Post Type UI) which will be the Holiday code.
Then in your permalink-settings make sure you have a custom structure of
/%category%/%postname%/
That’ll lead to the permalink-structure you’re looking for and also let you have archives of holidays within a specific holiday code. Win win.
A bonus is that this permalink-structure is recommended as the best for SEO.
