“To take part in the ESO Plus Bonus Event, log into the game with an active paid ESO Plus membership (the free trial does not count, sorry) each day from July 10 until July 15. Note that in order to qualify, each unique login must occur within a 24-hour time period that starts at 10:00AM EDT and ends at 10:00AM EDT the next day. If you remain logged in over a two-day period, it will only count as a single entry. You must log completely out of the game and back in – only logging out to the character select screen will not work. The free Crown Crates will be delivered to your account by Friday, July 20. Check your Crown Crates menu!”
If you take advantage of the free-play period, know that you’ll be able to check out basically all of the DLC and Morrowind (but not Summerset). You’re getting all of the perks associated with subbing with the exception of the monthly stipend, so that means the experience bonus, costume dyeing, double bank, and so on.