They will contact you
Hi,
My mother's confirmed interview date was 10th Jan, but I had requested for waitlist date as Dec 20th. They sent me an email saying that, there was slot open for that date, and if I was interested, I shd reply within 24hrs that the person is interested in attending the interview on that date, then they will send back the email with the confirmed date and time.
Hope this helps. Btw, her visa was rejected on grounds of 214b
Thanks,
Veeren