All World of Warcraft servers will undergo a 1-hour scheduled maintenance on Tuesday, September 2nd at 7:00 A.M. PDT (September 3rd, 03:00 CEST for EU servers). During this time, the game will be unavailable to play.
U.S. Realms Maintenance – September 2nd
Americas servers for all versions of World of Warcraft (Anniversary, Mists of Pandaria, and The War Within) will undergo maintenance at 7:00 AM PDT and are expected to be unavailable for approximately 1 hour, until 8:00 AM PDT.

E.U. Realms Maintenance – September 3rd
European servers for all versions of World of Warcraft (Anniversary, Mists of Pandaria, and The War Within) will undergo maintenance at 03:00 CEST and are expected to be unavailable for approximately 1 hour, until 04:00 CEST.

Source: Weekly Maintenance Schedule