https://www.techradar.com/news/why-we-think-amazon-prime-day-2018-will-be-on-july-10-and-what-to-expect

Just when is Amazon Prime Day 2018? Well, despite it being set to be Amazon's biggest day of the year, yes bigger than Black Friday, Amazon hasn't yet officially announced the date. But we're willing to bet our reputation on a date given what we know from past Prime Days.

Essentially, the last two annual Prime Days have landed on the second Tuesday of July. The first Prime Day was on a Wednesday (July 15 2015) though, just to put a slight kink in our theory, but Amazon seems to have settled on the Tuesdays in recent years, so that would mean Amazon Prime Day 2018 will land on Tuesday July 10.