Amazon's Prime Day is now, technically, two Prime Days. 

Amazon said that it will extend this year's Prime Day -- an annual event in July -- to two days. The shopping holiday, which will include "more than one million deals globally" according to Amazon, kicks off at midnight ET on July 15, and will run for 48 hours. Amazon shares closed 1.86% lower on Tuesday. 

The 24-hour extension of Prime Day is likely to turbocharge what was already a popular, and highly lucrative, event. Despite website glitches that stymied many shoppers in the first few hours of the event, Prime Day 2018 still drove roughly $4 billion in sales in 2018 -- a 65% increase over the prior year, according to Internet Retailer. 

Prime Day may be a multibillion-dollar sales haul, but it isn't the biggest shopping event out there. Prime Day is dwarfed in scale by Alibaba's (BABA - Get Report) Singles Day, a 24-hour event that brought in more than $30 billion in gross merchandise volume in 2018.

But for Amazon, a Prime Day sales windfall is just one small piece of the puzzle. It also serves as a "dress rehearsal" for the holiday shopping season, a strategic push to populate more homes with Echo devices, and a chance to deepen its sales leverage in households. 

"This is yet another example of the flexibility Amazon has to push the pricing envelope, in this case to broaden and deepen the benefits to Prime members, which creates a conundrum for all retailers, with the smaller, financially weaker retailers the most at risk," said Charlie O'Shea, a retail analyst at Moody's.

Amazon doesn't regularly disclose how many Prime members it has -- CEO Jeff Bezos wrote in an April 2018 shareholder letter that it had surpassed 100 million members worldwide -- but membership growth has decelerated since 2017, when an explosion of growth in Prime memberships launched the service into over half of all U.S. households. Based on a recent consumer survey, Evercore ISI estimated that Prime membership growth is now below 10%, and various analysts have pegged the U.S. household penetration of Prime at around 60%. 

Amazon would likely prefer that be 100% in time: Back in 2016, at a shareholder meeting, CEO Jeff Bezos defined Amazon's goal as "to make sure that if you are not a Prime member, you are being irresponsible."

In the meantime, the average annual spend per Prime member is creeping up. That figure could reach $2500 per member in 2019, according to Evercore ISI's research -- and Amazon's investment in one-day shipping, a new standard for Prime members expected later this year, could push that number even higher.

A recent study by Feedvisor, an intelligence platform for online retailers, found that nearly half of Prime members (45%) reported buying on Amazon at least once per week. About half (47%) also anticipate making a purchase during Prime Day. As Amazon broadens the benefits to Prime membership, it's likely to cultivate more highly active shoppers -- and attract new ones, too.

"Amazon's one-day shipping program will enable the company to increase their competitive edge versus other online marketplaces, and may also incentivize prospects considering Amazon Prime to join the program," said Feedvisor CEO Victor Rosenman.

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