Dungeons And Dragons Online: Server Downtime for Game Update: Wednesday, May 23 from 7am-1pm EST

From the launcher and official forums:

The game will be going offline Wednesday, May 23 from 7am until 1pm eastern, for Update 4.1: Mark of the Dragon

During this time all game servers except for Risia will be offline.

Release notes and known issues will be posted when the update is complete.