Philadelphia ? Retailers want shoppers to think about the ghost of Christmas future even before they deal with the ghosts and goblins of Halloween present.

Department stores already are stocking shelves with Christmas merchandise, in some cases setting up Christmas trees and holiday lighting.

Wal-Mart Stores Inc., Target Corp. and J.C. Penney Co. are some retailers hoping to get consumers into a Christmas shopping frame of mind two months early. Sears Holdings Corp.’s Sears and Kmart stores kick off the Yuletide mood in late October or early November. Costco Wholesale Corp. said it has always put out Christmas ornaments, gift wrap, cards and artificial trees as early as September.

“Although our busiest Christmas sales weeks will occur in November and December, we have been placing a selective sampling of our Christmas items into stores early in the season for several years,” said Tara Raddohl, a spokeswoman for Bentonville, Ark.-based Wal-Mart.

The trend began about three years ago, but more retailers are following suit, said Britt Beemer, chairman of America’s Research Group, a consumer behavior marketing firm in Charleston, S.C.

It’s a phenomenon called “Christmas creep,” according to the Wharton School at the University of Pennsylvania in Philadelphia. Hoping to catch early shoppers, retailers are extending their all-important holiday shopping season, which accounts for 25 percent to 40 percent of the year’s sales.

Shoppers at a Costco in Brooklyn, N.Y., pass a Christmas holiday display in October. It's beginning to look a lot like Christmas in many stores, and Halloween hasn't even arrived.

“The creep has been going on,” said William Cody, managing director of the Baker Retailing Initiative at the Wharton School. “Every year, retailers hope that coming out early is going to reduce the amount of promotions. They’d rather people buy at full price.”

Shoppers who spot a Christmas item they fancy might not want to wait for the product to go on sale a month later for fear it could be sold out. So they’ll buy it at full price, Beemer said.

It’s still not clear whether the strategy adds significantly to profits, Cody said.

The National Retail Federation, an industry trade group in Washington, D.C., said 40 percent of consumers plan to start their holiday shopping before Halloween this year.

“The demand for holiday merchandise is there,” said Kathy Grannis, the group’s spokeswoman.

As Christmas comes ever earlier, it’s given Thanksgiving merchandise the boot.

“Thanksgiving items are really something of the past – the pilgrims, the turkeys,” Beemer said.