Daytime players will be happy to hear that most realms are only getting rolling restarts. They'll be started at 5:00 AM PDT on North American realms and 10:00 AM PDT for Oceanic realms. There are a fair number of servers being hit with a real maintenance period, but it's only expected to take a few hours: Starting at 5:00 AM PDT and lasting until 8:00 AM PDT. Those realms are: Area 52, Arena Tournament 1, Arena Tournament 2, Auchindoun, Azuremyst, Blade's Edge, Blood Furnace, Coilfang, Dawnbringer, Exodar, Fizzcrank, Galakrond, Ghostlands, Grizzly Hills, Shattered Halls, Terokkar, The Scryers, The Underbog, Velen, and Zangarmarsh.
If you're not sure what those times are in your time zone, the World Clock is a pretty simple tool to help you figure it out.
As always, WoW.com will be here throughout the morning and afternoon should anything come up during maintenance. It's not a patch day, so we can be reasonably sure nothing will come up, but you never know.