Scheduled maintenance usually occurs on Tuesdays around 16:00 Pacific time.
Steam is a worldwide service and if you look at Steam’s player statistics page you’ll see that peak time for the service is during the European evening, and at 16:00 Pacific time the user numbers are significantly lower. Additionally Valve is located in the Pacific timezone and it sensible to do all routine maintenance during business hours so they can have their employees able to respond quickly if something goes wrong . See also: Valve’s FAQ on this topic.
Scheduled maintenance usually lasts around 10-30 minutes.
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