This Wednesday there will be an extended maintenance to apply patch 2.3.2. Please note that all European realms will be offline from 03:00 until 11:00, Paris time (CET).
Since we've never seen a patch go live in Europe that hasn't gone live in the US, this is pretty strong evidence that today is patch day. Patch 2.3.2 will include some class tweaks (primarily for Hunters, Mages, Shamans, and Rogues), some UI changes which may mean you have to pick up updated addons, bug fixes and more. See the full patch notes (linking to the PTR notes, as the live notes aren't up yet) for official details.
Update: The patch is now downloading for US players.