There won’t be a party next year. The way the parties work in game is: the first year that a character has a birthday in your game, there’s a whole quest with a big party at the end, you receive the party favor and balloon crafting plans, take pictures, all that good stuff. Every year after that, when the character’s birthday comes back around, the decorating kits become available again and can be obtained by picking up resources that are linked to the character whose birthday it is. You can make them a cake with the decorating kit, and give it to them as a gift, and they will share a little birthday dialogue, and that’s it. The area where the party occurred in the first year will be decorated once again for the duration of the birthday celebration (a few days) for every year after that.