Server maintenance has for "a 3.5 balance update" has been scheduled for April 10th at 2:00 AM PST for NA, 4:00 GMT for EUW , and 3:00 GMT+1 for EUNE.
Here is the Riot NOC announcement for NA:
"Summoners,On 04/10/13, starting at 00:30 PST, ranked queues will be disabled in preparation for a 3.5
Riot NOC
balance update. At 2:00 PST, the servers will be shut down and all games currently in progress will end in a draw and stats will not be recorded. We estimate the battlegrounds will be unavailable for 6 hours."
For the moment, It's unclear what exactly this patch will contain. As noted a few days ago, the PBE was "rolled back" to what is currently on live plus Karma balance changes and the replay system. Based on this, my assumption is this will not be the Freljord update with all the relaunches, Howling Abyss, and Lissandra.
