More than 2,300 merchants plan to take part in Free Shipping Day tomorrow, the fourth annual marketing event designed to give e-retailers a late-season holiday boost by ensuring consumers of delivery of their products by Christmas Eve.

According to FreeShipping.org, which organizes the promotional push, the free shipping offers will be revealed at that web site at 12 a.m. Eastern time Friday. As of Thursday morning, 2,339 retailers had said they would take part. Last year, 1,750 retailers participated in Free Shipping Day.

Last year, online retailers took in $942 million in sales on Free Shipping Day, up 16% from the same day in 2009, according to comScore Inc., though the web measurement firm did not directly attribute the increase to the promotional effort. So far this holiday season, e-retailers have recorded healthy sales gains over 2010, with increases of about 15%, according to multiple comScore estimates.

Friday’s marketing event is sure to resonate with many online shoppers, according to survey data from consulting firm Deloitte. Of the 5,000 consumers it surveyed earlier this year for its look at holiday shopping habits, 69% said they are more likely to shop from online retailers that offer free shipping. “48% of consumers say low price is the main reason to shop a particular online retailer this holiday season,” the report continues, “and 20% put free shipping at the top of their list.”

Deloitte also found that 46% of survey respondents will buy more items that qualify for free shipping this holiday season.