Happy Unbirthday!
This idea is taken from the Disney movie Alice in Wonderland. If you haven't seen it, you just need to know there's a hilarious tea party scene where two crazy characters, a rabbit and a guy with a strange hat, celebrate that it's their unbirthday. Thinking about it, it's better to celebrate your unbirthday since you have 364 celebrations a year!
Why not organize a surprise party for our partner as if it were their birthday? It doesn't matter if it's not; you don't need many reasons to gather some friends, invite them to a little something, and have a good time. Don't worry if there aren't many gifts... you can always ask them to bring a small token as a contribution to the party :)