Amazon has lost market share for the Prime Day shopping holiday it invented. Walmart is setting itself up for a bigger slice of those sales.

• Walmart once again is looking to compete directly with Amazon's Prime Day.
• The retail giant is holding 'Walmart Plus Week' through July 13.

Prime Day, the virtual shopping holiday that Amazon invented in 2015, has transformed July from a slow time for retailers into one of the hottest sales stretches of the year.

But Walmart isn't letting its e-commerce rival go unchecked, especially with consumers presumably in a buying mood.

Walmart and other retailers are angling for some of the billions of dollars that shoppers are expected to spend around Prime Day, and according to one study, they're starting to chip away at Amazon's market share during the critical sales period.

Prime Day will take place on July 11 and 12 this year. Walmart's answer is Walmart Plus Week, which runs between July 10 and July 13. While the first 24 hours of Walmart's event are exclusive to Walmart+ members, anyone can access the deals on the second and third days.

In addition to luring away potential Amazon shoppers, Walmart's event is geared toward getting customers to sign up for Walmart+ memberships.

Walmart did not directly answer when Insider asked about the timing of the sales week, but said that such events "allow us to celebrate our members in a fun way." In previous years, the company held "Rollbacks and More" events in July to compete with Prime Day, and last year Walmart launched a summer savings event called Walmart+ Weekend which took place just before Amazon Prime Day.

But while other retailers can try to get in on the shopping spirit, it'll be hard to beat Amazon at its own game.

Prime Day drove around $7.3 billion in sales for Amazon last year, according to Insider Intelligence. This year, the retail giant is expected to eclipse its old record with around $8 billion in sales.

According to Insider Intelligence analyst Andrew Lipsman, once a "shopping tent pole" like Prime Day is established, it's difficult to dislodge that in the minds of consumers.

"If you can't beat 'em, meaning Amazon, join 'em," said Lipsman, who added that it makes sense that big box retailers like Walmart and Target run concurrent promotions because customers are looking for deals during this time and don't necessarily care which retailer they get them from.

"Everyone is soaking up some of that incremental activity. But the ones who are best positioned to compete with Amazon are going to be your most well known big box retailers: Walmart, Target, and Best Buy," he said. Insider Intelligence is a sister company of Insider.

Billions of dollars in sales could be up for grabs during Prime Day

In 2022, other retailers competing with Prime Day made a collective $4.9 billion in sales during their Prime-alternative shopping events, according to a report by Insider Intelligence. While Amazon eclipsed that, with $7.3 billion in sales, competitors appear to be closing in.

"Amazon's Prime Day market share will continue to slip, dropping from 62.0% in 2018 to an expected 59.6% in 2023," the report said.

Lipsman added that Walmart and other retailers can lean into their strengths to compete most efficiently. For example, while Amazon's flagship category is electronics, Walmart may perform better relatively on everyday essentials like groceries and household purchases.

"The more that they lean into those offerings, the more they'll drive sales growth on that day," he said.

Roots of Walmart-Amazon Prime Day rivalry

The yearly battle over customers between Walmart and Amazon's Prime Day could date as far back as 2015. At the time, Jet.com, a buzzy e-commerce company cofounded by entrepreneur Marc Lore and eventually acquired by Walmart in 2016, was preparing to launch. Just weeks before Jet launched its website, Amazon held its first Prime Day — offering huge discounts to tens of millions of Prime account holders.

Jason Del Rey, author of "Winner Sells All," a book detailing the battle between Amazon and Walmart, said that Jet executives he spoke to wondered if the timing of Amazon's first-ever Prime Day "had anything to do with their impending launch." Amazon executives deny that it did, but in the time following Jet's launch, Amazon's pricing technology "followed Jet prices both up and down," according to the book.

Walmart isn't the only retailer to try to compete with Amazon's event. Target has Target Circle Week, exclusive to Target Circle members, which started July 9 and runs through July 15. Best Buy has a "Black Friday in July" savings event from July 10 to July 12.

"I don't think it's about necessarily dethroning Amazon," Lipsman said. "It's about getting whatever bump of activity you can."