In just a few short years, Amazon (NASDAQ: AMZN) has managed to create its own holiday. The online retailer has taken the doldrums of July -- a time when few people had much incentive to shop -- and turned it into a sort of second Black Friday.

This year's Amazon Prime Day will expand upon the previous three in both time and scope. Amazon websites around the world will offer a total of more than 1 million deals -- a 50% increase from 2017 -- during a period that will start at noon PT July 16, and run through midnight PT, July 17 -- 36 hours, compared to 30 last year.

It can be tempting to rush in, see the special deals, and spend some money. But impulse shopping on the site even on the much-hyped sale day could be a mistake, as not every Prime Day deal is a good one, and buying something you don't need (and may not use) is never a good idea.

Amazon started Prime Day in 2015. Image source: Amazon.

Do your homework

To take advantage of Prime Day, you need to be a member of Amazon Prime. The service, which includes free two-day shipping on millions of items, costs $119 a year (recently raised from $99), but non-members can sign up for a 30-day free trial. (Naturally, the company will automatically convert it to a paid membership and start billing you at the end of that period, but you can cancel it before that happens.)

Amazon offers members a sneak peek at a number of Prime Day deals, as well as a guide to shopping during the event. The online retailer has suggested that members download its app, and emphasized that Prime Day will not be limited to Amazon.com this year -- there will be deals at Whole Foods as well.

There are also pre-Prime Day deals that have already launched, as well as a number of offers for the big day that Amazon has released to members. Traditionally, Prime Day has been the best time to purchase Amazon-branded products like the Echo, Kindle, and Fire TV.

Use all the deals

While there are a number of offers and markdowns for Prime members who shop at Whole Foods during what the company is describing as Prime Week, the highlight among them is that Prime members who spend $10 the grocery chain between July 14 and the end of Prime Day will get a $10 credit to spend on Amazon.com.

In addition, Amazon Prime Rewards Visa cardholders will enjoy double their normal rewards, getting 10% back on purchases up to $400 from July 14 through July 17. Prime members who have not tried Whole Foods' delivery service will get $10 off their orders, plus $10 to use on a future order if they shop before the end of Prime Day.