Here is the Riot NOC announcement:
"Summoners,Note: There is no mention of new content AND there is no mention of maintenance for EU or the other platforms.
On 8/19/2013, starting at 10:30pm PDT, ranked queues will be disabled in preparation for a server maintenance. At 12:00am 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."
Update: The in client messaged now says in prep for the upcoming patch.