The heat is on for Amazon.

And the web giant’s second “Prime Day” today started out with some troubles that only upped the pressure. A “glitch” in the checkout process had Amazon trying to calm shoppers and tweeting, “Some customers are reporting difficulty with checkout. We’re working to resolve this issue quickly.”

Amazon was offering what it billed as up to 60 percent off athletic shoes and up to 50 percent off Fossil smartwatches.

“Prime Day” — which launched last year and celebrated the company’s 20th anniversary — is dedicated to deals and discounts for Amazon Prime members. Last year, it raked sales of approximately $400 million and also upset some customers who complained about products being sold out. This year, Amazon is expected to double last year’s sales and reach up to $1 billion.

The company has also boasted that it would have twice as many deals, although many of the discounts focused on Amazon’s own products.

This marks what some see as the first major sales event happening primarily on mobile.

“Prime Day is the first mobile-first loyalty event ever, let alone one at such a massive scale,” said Demandware and NewStore founder Stephan Schuambach. “It’s strikingly clear that Prime Day is built mobile-up, with deals, gadgets and content all focused on mobile consumers. Amazon is pushing its shopping app as the hub for Prime Day, which it continues to use as a fundamental tool to increase customer loyalty and therefore revenue.”

It also has spilled over into other retailers, in the vein of Hallmark’s Valentine’s Day and Alibaba’s Singles Day. Already, retailers including J.C. Penney, Bon-Ton, Wal-Mart and offering discounts and free-shipping. Unlike Valentine’s Day, they don’t refer to the offers as “Prime Day”; Penney is going with “Penney Palooza,” for example.

Amazon does not reveal the date of Prime Day until a month or less before the sales day, which might be an effort to prevent other retailers from planning competing sales in advance.

In a research note, J.P. Morgan said this was a key test for Amazon’s fulfillment and distribution operations in advance of the holiday season. J.P. Morgan also said that the Prime ecosystem continues to drive strong top-line growth.

Prime members pay $99 a year for benefits such as free two-day shipping and streaming of movies and TV episodes, and last year was the single biggest sales day for Amazon — including the holiday shopping season.

“We are always working to make sure customers find incredibly low prices on Amazon. Prime Day is designed to go above and beyond for our Prime members,” said Amazon Prime vice president Greg Greeley. “While the inventory behind our Prime Day deals this year is massive, at these prices, we expect many deals will still sell quickly.”

He also emphasized that Amazon was focusing many of its sales on Amazon-created products, saying, “I am especially excited about the deals on Amazon Devices like Amazon Echo, Amazon Fire TV, Fire TV Stick and Fire tablet.”