Marketers looking to make the most of their winter holiday online advertising campaigns should not overlook the month of October, according to recent research from AdRoll.

The report was based on 2017 data from customers who use AdRoll's e-commerce advertising platform.

Although more than 40% of US consumers begin holiday shopping before November, the average cost per click (CPC) for online ads is still 12% lower in October compared with November and December.

The average cost per thousand impressions (CPM) is between 15% and 20% lower in October compared with November and December, the analysis found.