Diablo 3 servers are scheduled for maintenance downtime on Tuesday, July 31st, which will last for eight hours. Quoting from the Battle.net forums:
We will be performing scheduled maintenance on Tuesday, July 31st. Maintenance will begin at 3:00 AM PDT and conclude at approximately 11:00 AM PDT. During this time, all servers and many web services will be unavailable. Thank you for your patience.
Source: Battle.net Forum