Learning from last year's mistakes, Amazon is placing more "top quality" items on sale and broadening the day's global appeal.
Amazon devices such as the Echo voice-enabled speaker, Fire TV and Fire tablet were available at big discounts as were a host of other products from high-end televisions to shoes and toys.
Not to be outdone, Walmart is offering five days of free shipping as well as a roster of discounts for all shoppers, not just members. This year, the e-commerce giant is expected to double its sales from Prime Day 2015, up to an estimated $1 billion.
Amazon said it was a success, with 18 percent more orders placed that day than the prior year's Black Friday, the shopping day after Thanksgiving that's typically the busiest day in retail.
According to analysis from Clavis Insight, consumer electronics is the most popular category among Prime Day "spotlight deals", promoted at the top of the page as especially in-demand items.
New deals will frequently roll out throughout the day so don't forget to bookmark and save items you're interested in to come back to later.
Amazon acknowledged the troubles in a tweet saying it is working to fix the problem quickly.
Wal-Mart also has discounts this week and recently began offering free 30-day trials of ShippingPass, its $49-a-year unlimited two-day shipping program.
Last year's inaugural event is reported to have produced $400 million in revenue for Amazon.
Start early for the best deals.
Buzz about Amazon's second annual Prime Day - the site's summer version of Black Friday - has been building for the past few weeks.
Amazon says it'll have almost twice as many TVs as it has on Black Friday and Cyber Monday.
So if you're one of those true blue Americans who firmly believe in capitalism, materialism, technology, widespread data collection, cheap labor or just that Entourage is the greatest movie of all time, Amazon Prime Day should bring tears to your eyes.