Bitcoin Pizza Day is not anniversary. But it was a memorable incident where Laszlo bought a large size pizza with 10,000 bitcoins. which is one of the most memorable events in the history of Bitcoin. Because of this, all bitcoin users commemorate this day with a pizza celebration. It introduces the event to new Bitcoin users and there is much joy to be had in the celebration. But if one wants not to celebrate it then there is no problem. But I think as a bitcoin user it would be great to celebrate such a milestone for bitcoin.