On Wednesday, July 15, new and existing members in the U.S. will find deals starting at midnight, with new deals starting as often as every ten minutes. They can shop thousands of Lightning Deals, seven popular Deals of the Day and receive unlimited fast, free shipping."
Amazon is even extending the festivities to new customers who sign up for the 30-day free trial of Prime, as long as they've signed up before the 15th. Amazon has promised Lightning Deals from nearly every category, so there could certainly be some great deals available.
Source:
Amazon