I'm fifty-fifty on whether PayPal counts as an official Valve representative at this point. It has a record of spilling the beans (opens in new tab) on Steam sales before Gabe and co. have a chance to do so themselves. Here, in a list of holiday deals (opens in new tab), we see the Steam winter sale pegged to start December 22, allowing us to cut it very fine with our Christmas shopping indeed.
Some of the capitalistic magic wore off in the autumn sale, when Valve announced (opens in new tab) that it would no longer be running flash sales and daily deals but instead adhering to a game's lowest price throughout, but there is a lot I haven't played this year and, worryingly, I have my card number memorised.
Thanks, Shacknews (opens in new tab).