Sales at U.S. brick-and-mortar stores on Thanksgiving Day and Black Friday were down slightly from last year, but the performance was still seen as strong in a holiday shopping season where discounts spread well beyond the weekend and many shoppers moved to the web. Online sales were up by double digits, according to data released on Saturday. Data from analytics firm RetailNext showed overall sales for both days fell 1.5 percent on flat customer traffic, while average spending per shopper dropped 1.4 percent.