The countdown to Amazon's 48-hour holiday has begun, with hundreds of Prime Day specials on Amazon hardware, including Echo Dots and Fire tablets, expected to be available when Prime Day kicks off on June 21.

However, during the past two Prime Day events we have found something interesting. Amazon is slowly adding more and more deals on non-Amazon devices. To be fair, Amazon has always offered sitewide deals on Prime Day, but over the past few years, our data team has been meticulously tracking Prime Day discounts and discovering trends that can help predict what to expect on Prime Day 2021.

Categories where the best deals are likely to be found range from everyday apparel to video games. Amazon devices will always come first on Prime Day, but if your home is already full of Alexa speakers, here are five categories worth your attention this Prime Day.

Shoe and sneaker deals are exploding on Amazon. Over the past two years, not only have shoe deals increased significantly, but the amount Amazon discounts from these products has also increased.

Last year, Amazon took up to 42% off shoes, according to our data, while 2019 saw deals of about 30% off shoes and sneakers. In other words, a deal that saved $11 in 2019 jumped to a $35 savings in 2020.

This trend is expected to continue in 2021, with some shoes already up to 50% off today.

Current deal: up to 50% off Adidas sneakers for men, women, and kids.

Over the past few years, Amazon has focused heavily on fashion. Brands such as Clavin Klein, UGG, and Levi's have vast storefronts on Amazon's website; Amazon also has its own brands such as Lark & Ro, Core 10 (in partnership with Reebok), and Daily Ritual. Core 10 even allows customers to create their own yoga pants.

So it is not surprising that apparel items during Prime Day jumped 12,113% from 2019 to 2020. And it's not just the number of deals that has increased. The average discount rate rose from 30% to 48%. As Amazon continues its expansion into fashion, shoppers can expect even more apparel deals for 2021.

Current deal: $15 off when you spend $100 or more on Amazon Wardrobe.

If you're one of the thousands of gamers who weren't lucky enough to get the PS5 restocked or the Xbox Series X restocked, here's some encouraging news. On Prime Day, the number of discounted PS4 and Xbox games nearly doubled. In addition, the average discount has risen to about $18 per game.

So if you still have a dependable PS4 by your side, now is the time to expand your game library. (For those of you lucky enough to have a PS5, check out our guide on how to play PS4 games on your PS5.)

Current sale deals: PS4 games from $9, Xbox games from $5

Whether you're planning a weekend getaway or a cross-country trip, the past two Prime Day events have seen a huge increase in luggage deals. According to our data, the number of Luggage Deals seen in 2020 jumped more than 950% compared to 2019. Discount rates have also increased, averaging 51% off last year (compared to 30% off the previous year).

Current deal: up to 48% off Samsonite luggage

Amazon is returning to its bookstore roots. According to our data, e-tailers offered more than 150% more discounted books in 2020 than in 2019.

The surge in book transactions is not that surprising given Amazon's start as an online bookseller. Additionally, as studios closed and sports leagues were forced to cancel games due to last year's shutdown, many people turned to books and reading as a form of entertainment.

Current promotion: 20% off books (Prime members only) [35]