Amazon had already revolutionized the way people read books, watch TV and shop online. And now it has succeeded in transforming the retail calendar.

Monday is the start of Amazon’s annual Prime Day sale, when subscribers to the company’s Prime service get major discounts on everything from flat-screen TVs to Ben & Jerry’s ice cream. Now in its fifth year, the sale has grown so large that the term Amazon Prime Day no longer quite captures it. This year’s Prime sale stretches across two days, and Amazon is not the only company involved. A record 250 retailers are offering their own sales to compete with Prime Day.

Analysts call this confluence of summertime promotions “Black Friday in July.” In 2018, retailers in the United States recorded $447 billion in July sales, $4 billion more than their total that December.

But the emergence of a second major shopping season — four months before Thanksgiving — may come at a cost for the hundreds of retailers scrambling to keep up with Amazon.