I too find this curious but I wonder -- is this something that's been done for others, maybe, in his AA group if he's in AA? So it's something that's not totally new? Will the people attending all be other recovering alcoholics so they "get it"? I'm assuming here that there won't be guests to whom his alcoholism or his sobriety are news -- right? It would be very uncomfortable for guests and especially for him if there are guests who come to a party and don't realize it's to celebrate sobriety -- or don't even realize that their friend/relative is an alcoholic.....So I'm going on the assumption here that the guests are all in the know and are people who have supported him as he gave up drinking--?
Is HE on board with this? I would not do this as a surprise party in case it backfired and he later said, "I really wish it weren't such a public big deal." Make sure he knows and embraces the idea of a party and the idea of being the center of attention like this. I have a friend who is a recovering alcoholic (20 years sober) and knowing her personality I feel sure she would not want an evening where the topic of discussion was her personal sobriety, even if the tone of the event were positive and supportive --she just would not feel it was a topic for a crowd unless the group was her AA group, maybe. But you know your husband best, and if he is on board here to make this a social event, I'd be interested to see an update on what happened.
I would have some party games that do not mention or reflect on sobriety or your husband but are just fun things to do with friends. A words of encouragement box where folks write notes to him on slips of paper sounds fine. Not sure about a toast and verbal congrats in front of the entire group unless that is totally fine with your husband.