These aren't necessarily announced the moment they happen on JP (as normally they overlap Urgent/Emergency Quests as their schedule is usually more packed), but they do post notices throughout the day that say that there will be a casino boost at x time. Usually there's also one shortly before.
I haven't seen the announcement system used yet on NA for anything other than Maintenance and Urgent Quest announcements, this would be a great way to utilize it especially because JP already does this.