Majority of U.S. households are Amazon Prime members

Amazon’s Prime Day began in 2015 as a sale day for Prime members, who were then a fraction of American consumers.

Now a slim majority of households in the U.S. are Prime members, said Andrew Lipsman, an analyst at New York-based eMarketer.

And 250 retailers are holding a mid-July sale.

Most of these retailers are shipping for free and most don't charge a membership fee. They want to prove that a Prime membership is not necessary to get a mid-summer deal.

Lipsman said stores also want to capitalize on the shopping event Amazon created.

July was a dull month for retail before Amazon convinced shoppers they could start their back to school shopping early. Now it's one of the biggest online shopping days of the year.