What is a decor pass and how does it work? I got a game mail saying happy birthday and that I'd received a decor pass but would like to know what they are talking about and whether it is redeemable only today, forever or for some period of time.

A Decor Pass is an item that you can use to get a free decor item from Barter. Go to Realm > Barter > Decor to pick what you want, you can use it at any time.

how do you assign the decor that you bought with the decor pass to to your wolf?

Go to the wolf you want to assign it to, then click "Manage Wolf" > "Edit/Add Decor".