Last year, Amazon (AMZN -0.17%) attempted to bring Black Friday, or at least a day when people get equally excited about shopping, to July.

Called Prime Day, the inaugural summer sale exclusively for Prime members was a mixed success for shoppers, but a financial home run for the company. Consumers were in some cases underwhelmed with the deal selection and lack of big-ticket, exciting items, but the bad buzz did nothing to stop people from buying.

During the one-day event, Amazon sold more units than it did on its biggest Black Friday ever. The company sold 34.4 million items across Prime-eligible countries, with 398 items ordered per second, according to an Amazon press release. The company said it had its single biggest day ever for Prime membership trial signups, though the company gave no specific number. 

The results were so successful that Prime Vice President Greg Greeley acknowledged that the big sale would become a recurring promotion. "Going into this, we weren't sure whether Prime Day would be a one-time thing or if it would become an annual event. After yesterday's results, we'll definitely be doing this again," he said in July 2015.

That day has come and the second annual Prime Day, which the company promised will be its "biggest Amazon event ever," takes place July 12.

Amazon Prime Day takes place July 12, 2016. Image source: Amazon.

How does Prime Day work?

The first thing -- and it's the most important -- is that the sale is only open to Amazon Prime members. That does not mean you have to pay for the $99-a-year service (which can also be purchased on a month-to-month basis now). Eligible people can use a credit card to sign up for a 30-day trial of the service which converts to a paid membership after that period, but can be canceled before you have to pay.

On Prime Day, members in the United States, United Kingdom, Spain, Japan, Italy, Germany, France, Canada, Belgium, and Austria, will find deals across nearly all departments and categories, according to an Amazon press release. Members in the U.S. can shop starting at midnight Pacific Time, with new deals as often as every five minutes throughout the day. 

Amazon has promised to have twice as many television sets on sale compared to Black Friday and Cyber Monday combined. In general, the company promises a better sale than last year, with Greeley quoted as saying:

Prime Day is a unique opportunity to discover new items and great deals, on top of our already low prices. Following last year's record sales, we have dramatically increased the inventory behind many deals. ... Even with this massive selection we know many of the Prime Day offers will sell out, so members should download the Amazon shopping app to receive notifications on their favorite deals.

Some offers are available during the whole day while others are released at timed intervals.

Why is this important for Amazon?

Amazon has done the near-impossible and turned the doldrums of summer into a shopping holiday that only it celebrates. The company has created a massive shopping event where consumers can only get deals on its website or through its apps. In addition Prime Day produces more than just one day of sales; it also engages existing Prime members (which might entice them to stay members) and it drives new membership.

That's very important for the company because Prime members spend more money on Amazon each year than non-members. Essentially Amazon has manufactured a faux holiday that pushes sales and sign-ups at a time when business would otherwise be slow.

Consumers may feel a bit manipulated by that, but there are good deals to be had, and a lot of money to be saved (and, of course, a lot of dumb purchases to make if you're not careful).