Black Friday has long been the favorite day of the year for retailers. This day after Thanksgiving is when ledgers traditionally switch from red to black ink as many stores’ negative balances turn to positives. Even before shoppers started waiting outside stores for hours in the dark just for a chance at one or two TVs or video games at ridiculously low prices, shop owners have counted on sales from this day to make their entire year.

Much more recently, the phrase Cyber Monday has entered the retail lexicon. This is the Monday following Thanksgiving weekend, when office workers who probably should be busy at their jobs are instead surfing the internet, ordering Christmas gifts from online vendors. In just a few short years, this shopping "holiday" has skyrocketed, with an estimated $887 million spent on this day last year by online shoppers in the U.S. for non-travel, retail items.

It’s time for shoppers to mark another day on their calendars—Small Bu...