On 09/09/09 I’m having a soiree, just because of the date. I need ideas! Obviously, we’re all going to dress to the nines and possibly listen to 90s music. Any other clever ideas?
Poker—9’s are wild
Both English and German versions of http://en.wikipedia.org/wiki/99_Luftballons
Sex party with 9" toys
Sports Jerseys featuring number 9 or 99
9-Ball tournament (billiards)
I like a date that means the same in US and UK format.
Perhaps something German-themed? “Nein, nein, nein!” I hear you cry.
Beethoven’s and Dvorak’s Symphony
Ying Yang decorations (derived from the associations with the Chinese dragon, represented by the number 9)
There are 9 muses in Greek mythology, 9 SCOTUS judges, and 9 circles of hell (according to Dante).
And when you’re ready to clear everyone out, just start playing the Beatles’ “Revolution 9”. 
You could celebrate my hubby’s birthday. He’s turning 51 that day.
Ah, the first time I’ve ever thought up a good use for 99 Bananas!
The movie 9 is scheduled to come out that day.
Commemorate the 100th anniversary of the death of Henri de Toulouse-Lautrec.
Celebrate the 100th anniversary of the birth of Carmen Miranda (there’s a costume theme for you).