FWIW, definitely put the date you can begin terminal leave as your availability date. In fact, and with a few notable exceptions (Delta comes to mind), the process is so plodding at many of these carriers right now that you are probably safe even going 4-6 weeks in advance of that. There is a lot of flexibility in class dates once you're hired, and it's not like they're going to withdraw the job offer if you can't be sitting in class on your official availability date. On the other hand, many of them will wait until a month or two before your availability date to interview you, and then by the time all the PRIA checks and background checks and decision boards, etc. are done, you end up blowing through most of your terminal leave waiting on a class.

Of course, if this makes you uncomfortable, do what feels good to you.