Prime Day is upon us once again. Amazon's (NASDAQ:AMZN) fourth annual shopping extravaganza will kick off Monday, July 16 at noon PDT (3 p.m. EDT) for an extended 36 hours.

Amazon uses Prime Day to boost revenue at a normally slow shopping time of the year: People go away on vacation, spending money on poolside drinks or cross-country road trips rather than the electronics, media, or vast array of odds and ends on Amazon's website.

But since only Prime members are allowed to take advantage of the deals on Prime Day, it's also a key driver of Prime membership growth, which is arguably Amazon's most important competitive advantage. Amazon recently announced that it had reached 100 million Prime members around the globe, a sign of the effectiveness of its loyalty program, which offers free two-day-shipping on millions of items, in addition to other benefits such as free video and music streaming and discounts at Whole Foods.

An ad for Prime Day showing an Amazon box holding various cartoon items including a rainbow

Image source: Amazon.

Not surprisingly, Amazon is promising that this will be the company's best Prime Day yet. Below is everything we know about this year's version:

  • For the first time, Prime Day will last 36 hours, up from 30 hours last year, essentially giving customers two days to take advantage of deals.
  • Also for the first time, Whole Foods will be included. Amazon closed its acquisition of the organic supermarket chain last August, so this is the first Prime Day the e-commerce giant has been able to include it. Amazon is offering an additional 10% off on hundreds of Whole Foods items that are already on sale. Deep discounts will be available on other popular products, and Amazon credit card holders will also receive double the rewards, or 10% back, on Whole Foods shopping from July 14 to July 17.
  • Prime Day won't just be available in the U.S. Deals will be offered to Prime members in 17 countries, including the U.K., Spain, Mexico, Japan, India, Italy, Germany, France, China, Canada, Belgium, Austria -- and for the first time, Australia, Singapore, the Netherlands, and Luxembourg.
  • Over 1 million deals will be available in categories, including TVs, smart-home items, kitchen, grocery, toys, fashion, furniture, appliances, back-to-school supplies, and everyday essentials.
  • Some deals are already available, including $100 off the Echo Show; deals are also offered on Amazon brands and on media like music, Kindle books, and video games via Twitch.

There are also some things we can expect from this year's Prime Day based on last year's results:

  • Prime Day is Amazon's biggest shopping day of the year, outpacing both Black Friday and Cyber Monday. Last year's sales increased 60% from the previous year, though Amazon, characteristically, declined to release a hard dollar figure. Considering Amazon is adding six more hours as well as Whole Foods items to the bonanza, this year's Prime Day sales could increase by at least that much over 2017.
  • Tens of millions of Prime members shopped during last year's Prime Day, a 50% increase from the prior year. Given increasing Prime membership, this year's event should see another increase in the number of Prime shoppers.
  • The most popular item last year was the Echo Dot. Considering the company's early promotion of the Echo Show, that Alexa-enabled gadget could be this year's biggest seller on the platform.

For investors, there are still a number of key questions about Prime Day that remain unanswered. We don't know how much revenue Amazon brings in from Prime Day, nor do we know how many members shop on the platform on Prime Day. And it's unclear how many Prime members Amazon adds thanks to the event.

Still, it's evident that Amazon sees Prime Day as a winner for both its customers and the business itself, as the company doubles down on it each year, expanding the number of hours, offering it in new countries, and making more products available for discounts.

It also looks like a success from a competitive standpoint, as many of Amazon's rivals like Walmart and Best Buy have followed its lead by offering their own shopping holidays to compete with the e-commerce giant.

While we many never know how much money Prime Day generates, it's clearly yet another way for Amazon to leverage its unique loyalty program. For Prime members and Amazon investors alike, that's reason to celebrate.