On December 10th at 3:00 AM PDT for NA, 5:00 GMT for EUW and 01:00 GMT for EUNE, servers will go down for maintenance and the 4.21 patch will be applied.
Here's NA version of the Network Operations maintenance announcement:
"On 12/10/2014, starting at 01:30 PST, ranked queues will be disabled in preparation for patch 4.21. At 03:00 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 3 hours.."Here's a collection of the patch 4.21 maintenance threads for each server:
I'll keep you updated as more information on what the patch contains and patch notes become available. While the latest PBE content is usually a good indicator of what's to come, please remember that it is tentative and not everything may make it to live.