When Wal-Mart starts its holiday markdowns three weeks before Thanksgiving you know it will be a tough Christmas season. The Arkansas-based discount chain a bellwether for U.S. retailing usually holds off on its ”door buster” sales until the day after Thanksgiving traditionally the year’s busiest shopping day. This year Wal-Mart decided that it couldn’t afford to wait. No wonder say scholars at the Wharton School and retail analysts. A host of economic worries -- plus concerns over toy recalls -- has this year shaping up to be a lump-of-coal shopping experience. Here is what retailers -- and shoppers -- can expect.