The UnConventions have always been whenever the local organizers want to host them. Not so with the Official Convention.
I don't see why it shouldn't be at a convenient date to the organisers, taking local conditions such as weather into consideration. If someone else complains and says it should be such and such a date and can't be moved, then they should volunteer to put on the event. I think the date should be up to those who have kindly volunteered to organise the event.