Cyber Monday, billed as one of the busiest online-commerce days of the year, is spilling into the rest of the holiday season as more consumers use mobile devices to shop whenever they please.

No longer waiting to return to work on the Monday after Thanksgiving to surf Web deals, consumers armed with tablets and smartphones are ordering online over a longer stretch, data from ComScore Inc. show.

That's spurring such e-commerce heavyweights as and eBay to grapple with retailers including Macy's Inc. and Sears Holdings Corp. in using online promotions and mobile applications to lure tech-savvy gift buyers long before and after Cyber Monday.

As more purchasing happens over the Web, e-commerce will keep outstripping shopping in brick-and-mortar stores. Online holiday sales will increase as much as 15 percent to $82 billion, more than three times faster than the total gain of 3.9 percent to $602.1 billion, according to the National Retail Federation.

"It now becomes more of a bell curve, where things start much sooner and end later," said Steve Yankovich, vice president of innovation and new ventures at eBay, which is adding features to its mobile app that tailor the shopping experience for individual users. "It's more fuzzy because we can shop so many more ways and have products ­fulfilled in more ways."

Evidence of an extended online buying period emerged last year, especially on Thanksgiving Day and Christmas Day, as consumers browsed online offers between post-turkey naps. Growth in spending on those days rose — up 32 percent on Thanksgiving versus 18 percent in 2011 — while the pace of Cyber Monday sales growth slowed to 17 percent last year, compared with 22 percent the year prior, according to ComScore.

The trend is set to accelerate this year as mobile offers take off. Commerce on tablets and smartphones grew twice as fast in the third quarter as desktop online spending, and Web users in August spent more time engaging with retailers on mobile devices than on ­desktops for the first time, according to ComScore.

The mobile wave is shifting the shopping behavior of people like Cristin Zweig, a 29-year-old communications manager at Trulia Inc. who splits her time between Chicago and San Francisco. While she used to focus her holiday hunting on Black Friday deals, Zweig said she may forgo that practice this year to take advantage of mobile ­promotions from her couch or the plane the following week.

"I used to look at Black Friday deals and see what was happening, but after this year I know I can get deals all the time, so I'm no longer concerned." said Zweig. "If you get 15 percent more off if you buy through your mobile app, that's a bigger incentive for me."

Today, one third of average monthly traffic for leading retailers — like Amazon, eBay, Wal-Mart Stores Inc. and Target Corp. — is from smartphones and tablets, according to ComScore. As a result, the stores are offering more holiday deals that can reach consumers whenever they're plugged into those devices. The holiday season is crucial for companies such as Amazon, the world's largest online retailer, which depends on gifters to fuel its biggest-revenue quarter.