This is a copy/paste from the realm status forums:
The following list of realms will be taken down tomorrow for a period of twenty-four hours starting at 5:00AM PDT. We realize that this is inconvenient; however, this downtime is necessary to address the recent issues adversely affecting performance for players on these realms. All World of Warcraft players on these realms will receive a one-day time extension for the downtime caused by this maintenance. All other realms will be brought back up at the normally scheduled time of 11:00AM. We thank you for your patience and understanding.