Green Monday, which is typically the second Monday in December, is feeling more and more like Cyber Monday in its early days — which means it’s an emerging opportunity for retail marketers.

Just like “Cyber Monday” was coined by the National Retail Federation in the 2005 to describe — and rally around — an existing spike in sales, the term “Green Monday” was similarly created by eBay a number of years ago to describe its best sales day in December. In recent years, some leading retailers including Walmart, Toys “R” Us, and eBags have picked up the “Green Monday” banner and tried to give it meaning for consumers.

“Green means go!” “Save some green.” “It’s like Cyber Monday again.” These and more have been used to describe Green Monday.

This year, 84% of the more than 100 major retailers that we track sent at least one promotional email to their subscribers on Green Monday, which was Dec. 8. That level of activity exceeds what we saw on Thanksgiving and puts it just behind Black Friday, which will likely hold its spot as the second busiest email marketing day of the year. (Cyber Monday is in the No. 1 spot.)

More than 10% of the email messages sent by the major retailers we track mentioned “Green Monday” by name. While we didn’t track mentions last year, anecdotally that’s a significant increase from a small base.

A number of retailers also made more subtle references to Green Monday, such as using green prominently in their email designs and working it into their discount codes. For example, J.Jill used the discount code “GMFREE” for their free shipping offer.

So while it’s still a minor occasion and still finding its identity, Green Monday seems destined for growth. I expect to see around 20% of promotional emails cite it by name next year.