Tomorrow is online retailer Amazon's new online shopping holiday: Prime Day. But Amazon, whose chief executive Jeff Bezos owns The Washington Post, isn't alone -- it's joining a long tradition of basically making up online shopping holidays.

Cyber Monday, the digital extension of the post-thanksgiving Black Friday weekend sales, was a term coined by the National Retail Federation's back in a 2005 news release: "While traditional retailers will be monitoring store traffic and sales on Black Friday (the day after Thanksgiving), online retailers have set their sights on something different: Cyber Monday, the Monday after Thanksgiving, which is quickly becoming one of the biggest online shopping days of the year."

It's gone on to be a pretty successful marketing gimmick --  becoming the heaviest U.S. online spending day and topping $2 billion in sales from desktop computers in 2014, according to comScore.  

But Cyber Monday sales pale in comparison Singles' Day, a joke anti-Valentines Day turned into a major online shopping day in China by e-commerce giant Alibaba. Last year, the company's sales reportedly topped $9 billion -- and founder Jack Ma told CNBC he hoped to take it global in the next five years.

Amazon Prime Day is a little different because deals will only be offered to people members of its Amazon Prime service -- a point WalMart threw shade at while announcing its own slate of competing sales.

But Amazon and Walmart aren't alone in trying to drum up business during summer months -- both Target and Best Buy have Black Friday in July type sales scheduled for this month.

And ultimately, selling more stuff is what pretty much all sales are about -- regardless of whether they come with their own made-up holiday.