The day after Christmas traditionally is one of the busiest for U.S. retailers as consumers return unwanted items and redeem an estimated $26 billion in gift cards to buy products still on the shelves. Total sales from "Black Friday" -- the day after Thanksgiving -- until midnight Monday were up 3.6 percent over the previous year, MasterCard SpendingPlus, a unit of MasterCard Advisors said. But retail sales, up 2.4 percent, appeared weaker-than-expected despite big markdowns just before the holiday, The Wall Street Journal reported.

Retail sales rose a strong 18.7 percent in the final weekend before Christmas compared to the final weekend last year, ShopperTrak RCT Corp. said. Sales were up a seasonally adjusted 2.8 percent in the week ending Dec. 22, the International Council of Shopping Centers/UBS Retail Chain Store sales Index, showed.

Economists had projected sales would increase 3.5 percent to 4.5 percent over last year but that didn't materialize despite strong sales at the start and finish of the Christmas buying season.

"If we didn't have those surges, it would have been a negative," Michael McNamara, vice president of research and analysis for MasterCard Advisors, told the Journal.