Actually there is always a celebration. Sometimes it is from Oct to Oct but most of the time it is for 18 months - don't ask me why I have no clue. Most of the celebrations are not a big deal or they do not do a big deal - you can see what celebration it is by the CM nametags
This celebration is the first HUGE one - really have gone all out! The other biggie was the millenium but I think this one is even bigger.