The fourth biggest retail event isn't even a holiday

Boxes move along a conveyor belt at the Inc. fulfillment center in Robbinsville, New Jersey.
Michael Nagle | Bloomberg | Getty Images

You could certainly call Jeff Bezos a master of invention.

Ahead of Amazon's second-annual Prime Day, a much-hyped sales event that offers deals to the company's subscription Prime members, new data from an online traffic and analytics firm found the made-up holiday generated the fourth largest amount of visitors of any day last year.

Prime Day brought 179 million visitors to the desktop and mobile sites of the top 25 U.S. retailers last July, according to SimilarWeb. That compares with 216 million on Thanksgiving, 219 million on Cyber Monday, and 248 million on Black Friday. Prime Day's numbers are all the more impressive in that they occurred in what is typically a slow month for retailers.

Yet Amazon wasn't the only retailer to reap the rewards of the event, which was originally designed to celebrate its 20th birthday. As a flood of shoppers took to the web to check out the deals and compare prices, competitors including Wal-Mart and Macy's likewise saw a lift in traffic, SimilarWeb's data found.

Similar to last year, when Wal-Mart ran a competing sale of its own, the world's largest retailer is promoting its online site ahead Tuesday's Prime Day. Last week, Wal-Mart said it would offer a free 30-day trial for ShippingPass, its competitor to Amazon Prime.

Neither Amazon nor SimilarWeb has issued a specific forecast for this year's Prime Day. But Amazon spokeswoman Julie Law said last week that the company expects it to be "another record-breaking Prime Day."

Amazon's second-annual Prime Day to launch July 12

Expectations for the event are high. Last year, Amazon sold more units on Prime Day than it did on Black Friday 2014, which at the time was its biggest ever. The event also led to more people trying Prime than any day in the site's history, Amazon said.

In an effort to top last year's results, the company will offer Prime members deals as often as every five minutes, an increase from every 10 minutes during last year's event. It's also promising to have deeper inventory levels, following customer complaints that last year's best deals were selling out too quickly. The company still plans on discounting quirky items, despite shoppers poking fun at several of them on social media.

And on Wednesday, Amazon sweetened the deal for shoppers in the 25 metropolitan areas that are eligible for its Prime Now service, which promises shoppers their products in an hour or less for $7.99. Some of its Prime Day deals will extend to these items, the company said.

Amazon's event is hardly the first made-up retail holiday. For instance, what shoppers now know as Black Friday didn't exist until the industry lobbied President Franklin D. Roosevelt to move Thanksgiving forward in 1939. The goal was to lengthen the holiday shopping season and try to capture more sales. And in 2005, the National Retail Federation created Cyber Monday in a bid to boost online sales.

Amazon isn't even the first retailer to invent a sales holiday. China-based Alibaba created Singles' Day in November 2009, an event that dwarfs the United States' biggest online sales days. The last Singles' Day pulled in $14.3 billion, the company said. That compares with Cyber Monday's $3.07 billion the same year, according to the Adobe Digital Index.

Although Prime Day has emerged as a top day for online retailers, it's worth noting that digital revenues still account for less than 10 percent of total retail sales.