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.

An Amazon Prime truck

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 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

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.

Don't be impulsive

Many Prime day deals won't be revealed until the event starts, and others will be unveiled at intervals during the sale. That strategy makes it easy for people to get swept up in the moment.

To avoid that, if you're planning to bargain hunt during Prime Day, start by making a list of things you intend to buy (and doing some pricing homework), and a list of things you might purchase if the deal is tempting enough. For example, having extra Amazon-branded charging cables is always useful if you own some of the company's devices, so you may want to pick up a few on sale even if you don't need them immediately.

But a bit of planning should help you exercise restraint in the face of Prime Day's frenzy of discounts, and prevent you from having to figure out what to do with the kayak or Ultra-HD TV you bought because it was such a great deal -- not to mention saving you from having to figure out later how to fit those bargains into your budget.