Pretty sure an Union event would require them to do some sort of maintenance beforehand, if only to make changes to the union system to prevent kicking/leaving at the very least. I also doubt they'll just randomly slap it in after a daily reset, without even a few hours notice beforehand.
But yea, they've most likely already added all the assets for the next events into the game, though it's still a toss-up as to when they'll actually start this next one. ¯\_(ツ)_/¯