That's the odd thing, it has almost always been major holidays or festivals. Feast of the Sail in February, for instance, has all kinds of similar counterparts in other worlds, so when Manaphy appeared and swapped people's bodies on that day, the connection was there. Then Children's Day, Tanabata, Harvesting Bonds... which leads me to believe that we're due for something strange, and it might be on or around Helixmas.