First, if you're next trip is Easter 2020, you've already missed the chance

You can schedule as far ahead as their calendar allows (I don't recall, offhand, just how far that goes). But keep in mind that given everything expect to have some troubles. My colleague applied in January of this year and didn't get his interview until early this month because of COVID, his appt was cancelled and rescheduled three times in Austin before he was finally able to get in for the interview, finger printing, etc.

But the TSA website has a calendar, and you should be able to reserve any date the calendar shows as open for whichever office you are doing the interview. You may also want to contact them and actually speak to someone on the phone. They were doing "walk-in" interviews at one point, IIRC, if you were making an international connection, so there may be a chance at being a bit more loose there, but you'll really need to speak to them for details on how all that works.

Sorry I can't tell you more, my interview was a few years ago now, and my renewal was simply filling out a form on the website and paying the fee.