Quantum Mechanics Day is December 14th. This is the day that, in 1900, Max Planck presented a theoretical derivation of his blackbody radiation law. This was the first formula to accurately describe blackbody radiation being emitted in quanta, and is one of the primary discoveries giving rise to the field of quantum mechanics.
Quantum Mechanics Day is generally not celebrated by much of anyone. It is largely a ride-along holiday to Agnostica, a somewhat silly agnostic replacement for Christmas. Agnostica was originally founded on the belief that Science beats Religion, and so was quite happy to pick up the 100th anniversary of Planck's discovery as an additional reason to celebrate in 2000. Since then a few celebrators have preferred to celebrate the rather laid back and value-neutral Quantum Mechanics Day rather than the often ideologically aggressive Agnostica. Sadly, is has not yet gathered the sort of following that Pi Day or even Square Root Day have garnered. This is a pity.
Quantum Mechanics Day has no real traditions or expectations, other than being a chance for one to be very nerdy and often somewhat tipsy. It would be an excellent theme for a nodermeet.