As of December 12, 2011 (which would have been the Fourth Day of Spiralmas according to the prior setup), the event has changed somewhat dramatically. This is primarily due to unforeseen academic issues on my part, and I apologize for this.
Now, the event is more simple than ever. Anyone can participate in this event simply by posting on this thread. Please note that the event is now based on 12 days instead of 11.
What should I have in my post?
Well, if you're familiar with the Christmas classic The Twelve Days of Christmas, you're all set! All you need to do is post a quick Spiral-Knights themed version of this song.
What you mean by "a quick Spiral-Knights themed version"?
What I mean is a shortened version with only the last verse, that has something to do with Spiral Knights. The obvious part is that you should replace all instances of "Christmas" with "Spiralmas". For example, consider replacing each of the 12 gifts in the original song with 12 Spiral Knights-themed items - or, you might instead (or also) consider replacing the "true love" part with an appropriate Spiral Knights figure (NPC, Monster, etc).
Sounds interesting!