Can be found here. Or copied below:
"The ground beneath your feet rumbles and shakes as Skaven tunnels have appeared all over the Old World. We are proud to announce the launch of Game Update 1.4 and The Verminous Horde RvR™ Pack. We will be bringing all North American and European servers offline at 7:30 AM EST/13.30 CET. At this time we anticipate all servers to be back online no later than 12:30 PM EST/18.30 CET. As always, please be sure to check the Herald for updates regarding this downtime."
And in case you've missed them, the patch notes are here.