On April 8th at 3:00 AM PST for NA, 5:00 GMT for EUW and 02:00 GMT for EUNE, servers will go down for maintenance and the 5.7 patch will be applied.
"On 08/04/2015, starting at 01:30 AM PDT, ranked queues will be disabled in preparation for patch 5.7. At 03:00 AM PDT, 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."Be sure to check your particular servers status page for your region's specific 5.7 maintenance times:
Things are a bit different this patch as the [Patch 5.7 notes] have already been published! Check out the official notes for a look at what is coming in the patch, including several new skins!