Should I archive my Holiday Products or mark Unavailable? How is the best way?
If you Archive or mark the products Unavailable, in that case the product pages will return the 404 error for the visitors that still happen to come across those product links.
In case some seasonal products will be discontinued, you can do a 301 redirect for those pages. More on this topic:
https://help.shopify.com/en/manual/online-store/menus-and-links/url-redirect
Alternatively, you can utilize the PushLast app to push that seasonal product to the end of the collection page.