Amazon.com Inc took a big gamble this year by expanding its annual Prime Day summer sale to four days from two, betting the extension would give shoppers more time to navigate the millions of deals on its sprawling web store.

Retail analysts say the first day of Amazon’s Prime Day event and competing sales drove solid online spending.

The event, first launched in 2015 as a way to hook new Prime members, has pushed other retailers to launch counterprogramming.