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.

Amazon Prime Day is usually met with competing sales from other retailers, and this year is no different. Walmart’s summer sale is back this year, competing with Prime Day and running for an extra two days.