I booked my interview on March 16 and I know that before that people booked on February 20 and around December 15, so my guess is that they open dates on the third week of each month. It took me almost two weeks of checking the website before I was able to book, I believe you should see dates available next week - but keep checking this week too, you never know...
I know it sucks to log in and see nothing, but don't give up, you're so close now! On the day I booked I checked at 9am and there was nothing, then at 3pm I decided to check again out of anxiety and there they were the available dates!! I almost fainted! 😂