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.

A Momentum Commerce researcher said the slow start could indicate “evolving consumer behavior” after Amazon doubled the length of its Prime Day sales.

A record $23.8 billion in sales is expected to be spent by consumers for Amazon Prime Days, Adobe said in a forecast released on July 7. That's a 28.4% increase year over year and $9.6 billion more than the comparable period last year. Last year, shoppers spent $14.2 billion during the two-day Amazon Prime event, according to Adobe.