Except for Xmas, I'd rather we avoid religious days. I daresay the majority of Hive is probably either Christian or from a predominately Christian population, but looking at it from the outside in, many may find a religious celebration uncomfortable. I have many Japanese friends, for instance, who find religious celebration uncomfortable. However harmless some of us think that may be, I don't think Hive should be in the business of turning off anyone. We want to attract as many people as we can to this network, not push any away by making them feel awkward. If you include one religious day (like, say, Easter) you need to include all religious days, and that would just be overwhelming, like trying to celebrate everyones' birthday.

Xmas is kind of an exception as it has expanded beyond religion and is now celebrated in a secular way in many places (like Japan).

My pick would be for fun, perhaps geeky, celebrations. Like pi day or star wars day (may the 4th be with you), talk like a pirate day, and these kind of fun ones that anyone can get into and are not culturally or religious exclusive.