I believe due to some unforeseen issues the Groove Coaster Collaboration and the Athena Descended dungeons (which were scheduled to start on 7/29) have been postponed. The Groove Coaster Zero dungeon will now begin on 8/12 and run through 8/25, and the Ancient Water Dragon (Bowmore) dungeon will take its place starting on 7/29.
As for the Athena Descended dungeon, a time hasn’t been given for rescheduling this dungeon yet and you should stay tuned for announcements regarding timing of this one.
A couple of additional tidbits – I believe on Twitter Murako announced that the leader skill of Athena will be 3x Attack for God type monsters.
Additionally, it looks like people are picking up that there will be an iOS update of the game coming today or tomorrow, as there is a new version update message for some of the time travelers. So be on the lookout for having to update your game on iOS (I believe Android already had their update).