Prime Day is an annual sitewide sale that Amazon puts on for members of its Prime subscription service. First held in 2015 in honor of Amazon's 20th anniversary (with mixed success), it was originally plugged as a "one-day-only event filled with more deals than Black Friday." In the years since, it's morphed into a 48-hour affair that's preceded by a few weeks of preview offers. "Prime Day" is a misnomer at this point.