New York ? Cyber Monday started as a gimmick to get people to shop at their desks on the first workday after Thanksgiving. But if you promote something enough, it can take on a life of its own.

This year, stores swamped customers with online ads and e-mail deals, and sales could top $1 billion, making it bigger than any single shopping day last year.

Online sales were already running 15 percent ahead of last year’s by 3 p.m. Monday, with the biggest shopping hours of the day still to come, according to IBM’s Coremetrics tracking service.

“The numbers are really strong,” said the service’s chief strategy officer, John Squire, who added that he expects Cyber Monday to be the biggest online shopping day of the season.

The Monday after Thanksgiving was dubbed Cyber Monday by the National Retail Federation trade group in 2005 to describe the unofficial kickoff to the online shopping season. The idea was that people returning to work after the long weekend would shop at their desks.

It never really was the busiest online shopping day of the year.

But like any good marketing angle, it spawned imitation. Nearly 90 percent of U.S. retailers offered some kind of Cyber Monday promotion this year, targeting shoppers who didn’t want to venture out at 4 a.m. for those in-store deals. In 2007, 72 percent offered a Cyber Monday promotion.

Rachel Bergman, general manager of e-mail marketing service Experian CheetahMail, said this year the company sent out several hundred million Cyber Monday promotional e-mails, 40 percent more than last year — on behalf of clients like Borders, J. Jill and Bass Pro Shops.

It has rarely been the biggest online shopping day of the holiday season, however — last year it was No. 2 — and this year, online deals have been stretched by retailers throughout the weekend.