The patch is a hefty one, clocking in at slightly over 1Gb, so if you haven't downloaded it yet, be sure to run Blizzard's background downloader and get all the necessary files before the big day. That way, you can hit the ground running as soon as realms come back online. Realms are scheduled to go down at 2 AM Pacific Daylight Time and should go up around 2 PM PDT. Don't worry too much about the exceptionally long 12 hour downtime. We'll keep you company.