Retail sales fell 4.1 percent in the week ending on Christmas Day, according to research firm ShopperTrak, largely because December 26 fell on a Sunday this year and was not included in Christmas week sales.

ShopperTrak said it still expects sales for the November and December holiday period to be up 4 percent compared with a year earlier.

The firm cited as evidence strong sales in November and on December 23. It also thinks that people who stayed home because of the snowstorm that struck the northeastern United States this week will go out and shop more in the coming days.

December 23 was the season's biggest shopping day after Black Friday, the day after the U.S. Thanksgiving holiday and the unofficial beginning of the holiday spending rush.

About $1 billion in sales that could have taken place on December 26 and December 27 were postponed because of the storm, ShopperTrak said. The snowstorm shut roads and airports in New York and created havoc across the Northeast.

The firm said that U.S. foot traffic on the day after Christmas was down 11.2 percent because of the storm.

Most analysts and research firms are expecting retail sales to improve from 2009's sales.

(Reporting by Jon Lentz and Phil Wahba. Editing by Robert MacMillan)