The downtime will happen from 5:00 a.m. PDT / 8:00 a.m. EDT and last until 7:00 a.m. PDT / 10:00 a.m. EDT. This of course assumes that there'll be no extended super-fracken-duper maintenance put on top of that. And we know just how much Blizzard loves to increase their maintenance periods. If there is you'll hear it here.
While the maintenance is going on, you can check out WoW.com's Guide to Patch 3.2 and read up on all the things you missed, or think you missed. There is also the beginnings of a list of bug fixes; a list that's sure to grow over the coming days.