Green Monday, the second Monday before Christmas and this year on December 13th, once laid claim to being the busiest online shopping day of the year. That is, until 2009 when it fell all the way to the 5th most important day (source) with the following Tuesday and Cyber Monday moving into the lead.
I expect similar data this year. The holiday shopping season got a strong early start with shopping on Thanksgiving, Black Friday and Cyber Monday all growing significantly year-over-year.
Why has Green Monday fallen? It's primary pitch came from retailers trying to scare customers with fears about shipping deadlines. If you look at the BradsDeals Holiday Shipping Deadlines chart, it is close to the last day to get free ground shipping but there is still plenty time to get free expedited shipping. Most retailers now provide free and fast shipping all the way up to the 23rd.
Most importantly, it is not a significant day for consumer deals. Patience is still the key. If you see a great deal, take it, but if you don't, there's still two more weeks to order online and still get free shipping at many stores.