I asked, and they said August 21. So basically, if you're ordering via JOD you can get the watch while you're returning your old phone!
You could get it right now from Samsung.com. But if you're trading in a Gear S3, I would check that the pre-order promo from Samsung is greater than it is for your Gear S3. I took the liberty of checking for you and it's $162 from Samsung, as opposed to $100 from Samsung. So basically if you can wait a couple weeks, you get a $62 reduction in the price of the Galaxy watch 2.
Here's where to check the return value of your T-Mobile trade ins:
https://www.t-mobile.com/resources/phone-trade-in