This is because Amazon has announced that for Prime Day, for a very limited time only, Prime members will be able to sign up for a Luna trial. This will be available on the 21st and 22nd of June and if you’re a Prime member, signing up on either of those days will get you a free 7-day trial to the service.
Once the trial ends, you will then have the option to keep subscribing to the service by paying $5.99 a month. Essentially what Amazon is doing is giving Prime customers a two-day window to get in on Luna, otherwise once that window closes, you’ll have to wait for an invite or wait until Amazon opens Luna up to everyone else.
If this sounds like something you might be interested in, then head on over to Amazon’s Luna page during those two days and sign up for an account.