Turbine published today that Book 13, Doom of the Last-king will be brought to the live servers on April 24:

The LOTRO Game Servers will be brought down on Thursday, April 24 from 6:00 AM - 12:00 PM Eastern (-5 GMT) for an update to Book 13: Doom of the Last-king. Release notes are now available on the Lorebook. Roheryn will remain up and available during the downtime. Thanks for your patience, and we'll see you back in the game soon!

In addition, for those that wish to, the update is also available for immediate download via torrent. You can find more information about this, and how to obtain the download, here.