Husky

That's kinda an interesting scenario really. The boss raids in-universe would be 'open' in that anybody can just waltz right in, but 'closed' in that everyday people aren't given the time and date where the groups meet up at the door. You'd actually need a reason to be at that boss door instead of just chilling in the wilderness in a way if you aren't in the main group of players meeting for the boss.

If anything, the Info Broker, or the Notice board should've inform all the player what time is it until the next Boss raid so everyone is ready for it. If that wasn't the case, then it wouldn't make sense for everyone just assumed that X was the day for the boss raid to start. That's too many coincident assumption to be true.