On September 25th at 3:00 AM PDT for NA, 5:00 GMT for EUW and 02:00 GMT for EUNE, servers will go down for maintenance and the 4.17 patch will be applied.
Here's NA version of the Network Operations maintenance announcement:
"Server Maintenance: Patch 4.17Here's a collection of the patch 4.17 maintenance threads for each server:
On 09/25/2014, starting at 01:30 AM PDT, ranked queues will be disabled in preparation for patch 4.17. 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."
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.