There are few celebrations that I can think of at the moment. It has been a very long time since I've had cause to observe any of them.

[ There's a pause as Megatron searches through his memory banks. It's been a long time. ]

There was Primus' Day, a celebration of the start of a stellar cycle and of the first god of our pantheon, if you were religiously inclined. When I was younger it was mostly an excuse to get together and have a party. Those sparked in creches or on the same day often held celebrations on the anniversary of such events. There was Senate Day, but I don't think it's celebrated any longer, since the senate is long defunct.