Amazon is making its "Prime Day" sale a four-day event. The yearly discount event began as a one-day sale on July 15, 2015, and has now expanded to four days. Related: Amazon Lays Off Workers in Its Book Division, According to Leaked EmailThe sale is only for Prime members, who pay $14.99 per month or $139 for a year of membership. Amazon says it has more than 200 million paid Prime members. Jamil Ghani, Amazon's vice president of worldwide prime, told Reuters that the company extended the number of days after listening to member feedback.