There will be two kinds of maintenance tomorrow.
The first kind will be from 3:00 a.m. PDT until 11:00 a.m. PDT. This is the "normal" maintenance time. This will apply to all realms that are not on the list after the break.
The second kind of maintenance will be from 12:00 a.m. PDT until 12:00 p.m. PDT. The realms after the break will be undergoing this maintenance.
It is likely the Wintergrasp changes we just broke will go live tomorrow. There's a bit of uproar about them, and it'll be interesting to see exactly what the changes are. There is no indication if patch 3.1.2 will be dropping tomorrow or not.
The list of realms down for half a day after the break.