U.S. retailers rolled out deals Sunday ahead of Cyber Monday in an effort to extend what was once a one-day shopping event and to bring in more customers, the Financial Times reported. The early Cyber Monday start comes as U.S. retailers battle for market share with Amazon and other online vendors. Cyber Monday sales in the U.S. are expected to reach $3 billion for the first time in 2015, up 12 percent from last year, according to projections by Adobe, Reuters reported.

The National Retail Federation trade group came up with the term "Cyber Monday" in 2005 when it detected a spike in online sales following the post-Thanksgiving Black Friday.

Walmart, Best Buy and Macy's were among some of the largest U.S. retailers to roll out deals early. Best Buy and Amazon, the world’s largest online retailer, are extending their Cyber Monday sales through the week.

Walmart, the world's largest retailer by revenue, has previously put up a handful of teaser deals on the Sunday after Thanksgiving last year, but this year marks the first time it will offer all of its Cyber Monday promotions the day before. It will launch 2,000 online-only specials at 8 p.m. EST.

Despite the deals beginnig Sunday, "nearly 80 percent of consumers we've surveyed still plan to shop on Cyber Monday," Artemis Berry, vice president of digital retail at the National Retail Federation, told the Los Angeles Times.

Online spending between Thursday and Saturday rose to $6.1 billion, a 19 percent increase from last year, according to the Adobe Digital Index. Adobe is predicting a $3 billion in online sales on Monday. 

Spending over the entire holiday period is projected to increase 3.7 percent to $630.5 billion this year, with the average family spending $805 -- below the 4.1 percent increase in spending recorded last year.