Helped by stores open at least a year, leading off-price retailer TJX (NYSE:TJX) reported on Thursday a 6% gain in December sales, pushing the company to raise its full-year guidance.

The Framingham, Mass-based apparel company posted sales for the five-week period ended Jan. 1 of $3 billion, up from $2.9 billion a year ago, driven by a 2% gain in comparable store sales, which is on top of a 14% increase in 2009.

Carol Meyrowitz, TJX chief executive, said the company was pleased with the results, as they significantly exceeded the retailer’s expectations for the period.

“Comp store sales continue to be driven by growth in the number of customer transactions as value, fashion, and great brands continue to resonate with customers,” she said. “We enter January with inventories well positioned to capitalize on the great opportunities we are seeing in the marketplace.”

Given the strong margins, the company, which operates T.J. Maxx, HomeGoods and Marshalls, raised its full-year earnings, now anticipating earnings in the range of $3.16 to $3.17 a share, representing a year-over-year increase of 20% to 21%.