Mondays are the busiest days for online holiday shopping during the season that runs from the Thanksgiving holiday to the end of the year, and advertisers should plan their spending accordingly, a study released on Thursday said.

Holiday shoppers tend to follow up their weekend store visits by researching comparative pricing and purchasing on Mondays, according to the study released by Atlas, an online marketing analysis division of aQuantive Inc.

Internet analysts expect online holiday sales in the United States to grow 20 to 30 percent this year as consumers are lured by free shipping, convenience and holiday promotions.

Atlas forecast that December 12 will be the peak Internet shopping day this year in what has become an estimated $26 billion business for online retailers like Amazon as well as traditional store chains such as Gap Stores.

Some good solid data is available in this reuters.com article supporting the peak shopping day scenario.