With the December sales pace slightly below normal, retailers relied on deeper discounts to help fuel a last-minute surge in holiday shopping, according to the National Retail Federation (NRF). Although final holiday numbers won't be reported until Tuesday, a December NRF retail executives survey showed slightly above normal readings in key industry operation measurements.

The Retail Sector Performance Index (RSPI) for December posted an above normal reading of 51.9-2.7 points lower than the prior month and 4.7 points lower than the same period one year ago. The RSPI measures retail executives' evaluations of monthly sales, customer traffic and the average transaction per customer, employment, inventories and a six-month-ahead sales outlook expectation. The RSPI is based on a scale of 0.0-100.0 with 50.0 equaling normal.)

NRF continued to project an increase of 4.5% in holiday sales over last year, bringing estimated revenues of $219.9 billion this holiday season. NRF will be releasing final holiday numbers on Jan. 13 in conjunction with the release of the U.S. Department of Commerce's retail sales report.