Certain Steam services will be unavailable starting at 4am PST on Monday, October 11th as Valve does maintenance on their servers. Estimated total downtime is listed as 4 hours.
Beginning at 4:00 am Pacific time Monday October 11th, we will be performing software maintenance on the Steam servers. We expect the resulting downtime to last four hours at most, putting the end of the window at 8:00 am Pacific time.
Some Steam services will be offline during this time. Affected services will include the Steam Community, in-game matchmaking or lobbies, downloading games from Steam content servers, and making purchases from the Steam store. Games that do not rely on Steam matchmaking services should remain playable.
We will work to make this downtime as short as possible. Thank you for your patience!
Update 1: Looks it might be up by 12pm EST. Keep your fingers crossed!
Hi all,
The updated estimate for the downtime is that we’ll be back online at 9 AM PDT. Sorry for the delays, and thanks again for your patience.
Taylor
Source: Steam Forums
Update 2: Reading and getting multiple reports of some users being able to access their Steam games while others are still having issues. Official word is now that all will be up by 1pm EST.
Source: Steam Forums