Strong post-Thanksgiving sales boosted retail stocks early on Monday, but shares fell later in the day as the overall market slumped and doubts blunted early optimism. Consumers poured into stores and flocked to online shopping sites on so-called Black Friday, the day after Thanksgiving that marks the traditional start of the holiday shopping season. Overall, retail spending for the weekend was up more than 12%, according to research firm ShopperTrak RCT. Wal-Mart ( WMT) and J.C. Penney ( JCP) especially reported strong sales. What resulted were modest gains in early trading. But many retailers gave up those gains in the afternoon as the overall market slumped on manufacturing news . Additionally, some analysts raised concerns about whether the retailers' final results would turn out as well as the early returns, given the short season and a trend toward heavy discounting that could eat into margins. The majority of holiday sales come in the two weeks prior to Christmas, wrote Kristine Koerber, a retail analyst with W.R. Hambrecht, in a research note. It's simply too early to tell how well things will turn out, she wrote. "Consumers typically wait until the last minute to purchase gifts, and we see this year as being no different." In fact, consumer shopping over the weekend was driven by the shortened holiday shopping season and a large number of discounts offered by retailers, said Scott Krugman, a spokesman for the National Retail Federation. "Retailers have done an amazing job of letting consumers know that they are all going to be discounters this year," Krugman said. Both Wal-Mart and Sears, Roebuck ( S) for instance, heavily promoted Friday discounts to customers. Although declining to give sales figures, Sears spokeswoman Peggy Palter said the company saw an "encouraging" start to its holiday season, with foot traffic in its stores as busy or busier than last year.