In case you are wondering what this is all about, I'll direct you to this related question: A happy Oli♦ day!
Basically, we are celebrating our beloved moderator Oli. It's not his birthday, as since rumors suggest, but rather the anniversary of his becoming an Ubuntu member. Or to put it another way, it's an excuse to have cake and party. Whatever works.
So, what do we do here to celebrate? I'm glad you asked! (You did ask, right?) It has become tradition to replace your gravatar with that of Oli's for the duration of the day. You can see that I have done this.
tl;dr — happy Oli♦ Day!