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.

Online spending is expected to surge to $23.8 billion across U.S. retailers during a 96-hour Amazon Prime Day event this week, according to an Adobe Analytics forecast released on Monday, as shoppers seek strong discounts on back-to-school gear ranging from apparel to electronics.