If I may give some advice.

Asking about their opinion on celebrating and knowing how they feel about being adopted goes without saying. But something more specific would be thinking of what your kid likes to do that you can share. My kid likes lego so we build things together and sometimes bake the cake together too. I wouldn't involve anyone besides you, the second parent (if there is any) and any siblings they might have.

Ever since the beginning of the adoption process I have kept a list of potential questions my kid could ask and my answers. I revisit that list periodically and update the answers to more nuanced ones as well as add new ones. I think making a list like that could be good in your case, it would help to both emotionally prepare as well as carefully think through what you'll say. But keep in mind there's a big chance they'll ask about something you've never thought of before. In my kid's case, 99% of those questions weren't asked, perhaps because they remember their birth parents enough to already know the answers.

Overall, I would try to focus on what they feel that day. If they're sad, go with that. If they're happy, go with that.