The fifth annual Free Shipping Day hosted by FreeShipping.org will be on a Monday—Dec. 17 to be exact—for the first time this year. The change will result in the event matching or beating the results from last year’s Cyber Monday, which refers to the Monday following the Thanksgiving weekend, predicts FreeShipping.org founder Luke Knowles. Last year Cyber Monday generated $1.25 billion in sales, making it the top online sales day of the 2011 holiday season, according to comScore Inc.

Mondays are a key day for targeting online holiday shoppers because that’s when consumers return to work after the weekend, time they may have spent shopping or discussing gifts with friends and relatives. Many office workers are in front of a computer much of the day, and can quickly make an online purchase.

Free Shipping Day is an event for consumers to shop online and receive free shipping with guaranteed delivery by Christmas Eve. Retailers sign up to promote their free shipping offers along with any promotions of their choosing on FreeShippingDay.com. The original goal in founding the one-day event, Knowles says, was to promote online shopping later into December than was common five years ago.

“One of the goals for us originally was that this would be a day that all online retailers would benefit from as it gets shoppers to spend money online rather than going to the mall,” he says. “The people who benefit most from this are not us, it’s the retailers. They are making tons and tons from sales—and a lot doesn’t even happen from our web site.” Knowles says many e-retailers, whether they register to appear on FreeShippingDay.com or not, benefit from the word-of-mouth and shopping frenzy the event generates online.

He picked Dec. 17 this year because it is roughly the last date when orders placed online can still be delivered in time for the holidays, he says. “This is just a big marketing push for procrastinators.”

Last year Free Shipping Day was on Friday, Dec. 16, and 2,600 retailers participated, Knowles says. He estimates at least 2,000 retailers will sign up this year, probably many more. Online sales on Dec. 16 totaled $1.072 billion, making it the sixth heaviest web shopping day of the 2011 season, according to comScore.

FreeShipping.org  works with more than 4,000 retailers to showcase their free shipping deals year-round. The company makes money from affiliate marketing, that is, when a shopper clicks from its site to make purchase it receives a commission, Knowles says, and its partners include Land’s End; J.C. Penney Co. Inc., No. 20 in the Internet Retailer Top 500 Guide; and Victoria’s Secret Direct, No. 18. Land’s End is owned by Sears Holdings Corp., which is No. 8.