Not everyone gets access to the thousands of deals being offered July 15 and 16 for Amazon (NASDAQ:AMZN) Prime Day. To shop on the big summer sales holiday, you need to be a member of Amazon Prime, which costs $119 a year or $12.99 a month.

You can, of course, work the system by signing up for a free 30-day trial for the service then cancel it before you get charged after the trial period. But in some cases, it's worth paying for a month or even a year of Prime membership just to get access to Prime Day.

What is Amazon Prime?

At the core of Amazon Prime lies access to unlimited two-day (soon to be one-day) shipping on more than 100 million items with no minimum order required. That's a powerful benefit that makes paying for Prime worth it for tens of millions of people.

Prime members also get access to a video service that offers a mix of originals and already-aired programming and a music service with a few million songs. On top of that, members get special deals on Amazon products, early access to sales, discounts at Whole Foods, and a lot of other perks.

Overall, it's fair to say Prime is a very good deal for anyone who buys from Amazon at least a couple of times a week in small quantities, and it gets better if you use the other benefits. Access to Prime Day also comes with membership, and that alone might make joining worth it.

An Amazon Prime truck.

Joining Prime gives you access to free two-day (soon to be one-day) shipping. Image source: Amazon.

Really, just for Prime Day?

Prime Day offers deals across a wide spectrum of Amazon's extensive product line. If you want to save on an HDMI cable or other accessory, the prices will be great, but there will also be big-ticket deals. It's access to sales on those more expensive items that may make the Prime equation work out for you even if you're only joining for access to the sale.

Amazon, for example, might put the $149 Echo Plus on sale for $99. Buy that item, and you've saved well more than the $12.99 a single month of membership costs, and you're nearly halfway toward offsetting the cost of joining for a full year. Buy a television, computer, or other big-ticket item, and it's possible to save enough to make your full-year membership effectively free.

Plan ahead

If you join Amazon Prime primarily to get access to Prime Day, make sure the amount you think you're saving is real. On Amazon-made products, the savings will be significant and not offered elsewhere. For other items, it makes sense to make sure that other retailers aren't offering similar (or better) prices on their Prime Day knockoff sales that don't require membership.

It makes sense to use Prime Day to offset the cost of a Prime membership if you're buying things you need that fit your budget. Doing that gives you access to the free shipping and other perks Prime members get for the length of your membership.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.