Be patient. Try again in a day or two. There were no critical OS X updates released in the last 24 hours.
Apple servers are incredibly busy because there are several hundred million iOS device users all trying to update their iPhones, iPads and iPod touches to the latest release for mobile devices, iOS 7, just released yesterday, September 18th, 2013.
Expect a similar flurry of server activity when the next Mac OS X, version 10.9 "Mavericks" is released, perhaps in October. Even though there are far fewer Mac users than iPhone users, major OS X release downloads are much larger.