P.B.
My mom called me on my 39th birthday & wished me a happy 40th. After my horror subsided she said, "See, 39 is not so bad."
So, you could either throw a 40th bash, or go the complete opposite route and throw a big "11th annual 29th birthday" party.
There is always the classic "funeral style" party where everyone dresses in black and the whole 9 yards...
Have Fun!