Maintenance for the week of July 13:
• PC/Mac: No maintenance – July 13
The issues on the North American and European PlayStation® megaservers have been resolved at this time. If you continue to experience difficulties at login, please restart your client. Thank you for your patience!
They never announce it ahead of time (for obvious reasons), but a tentative guess based on past examples would be April or August (and then December/January).
The sales have always been in April (around the anniversary), Auguts (Quakecon), and December (holiday sales). We can't bank on it, but it's likely we'll see that again.