December 22, delays are always possible but it's not going to launch earlier than that ("no earlier than" is often abbreviated "NET" in that context).

https://jwst.nasa.gov/content/webbLaunch/countdown.html

Well. It has shifted to the 24th now: https://blogs.nasa.gov/webb/2021/12/14/webb-space-telescope-...

As an extra question: when do we start getting data? Specifically, how long until we can start finding out if planets around other stars have oxygen, or even industrial pollutants like CFCs (my understanding is that if we see CFCs, it's almost certainly due to complex life and couldn't be a natural phenomenon. Oxygen is unfortunately more ambiguous, apparently).

Ever since I heard about JWST (I feel like it's been 8 years ago now), I was looking forward for an answer to this question. All the delays have been so painful...

December 22nd; they were aiming for the 18th but there was a small issue that required a bit of additional testing so they pushed it back to the 22nd.