Blizzard has announced in its official forum that the 1.04 update of Diablo III goes live later today, as servers will be going down to prep for it.
According to the post, the maintenance will vary according to region:
* The Americas: 3:00 a.m. PDT to 1:00 p.m. PDT (8/21)
* Europe: 8:00 p.m. PDT (8/21) to 6:00 a.m. PDT (8/22)
* Asia: 11:00 a.m. PDT to 11:00 p.m. PDT (8/22)