I would normally book the day before, due to the simple reason being , it is a guaranteed booking. I have had way too many experiences, in which I would call a w/g last minute or a couple of hours before only to be told that she is booked in the specific time I was looking to meet her.
So, my priority would be to book on the previous day
Also, I would suggest calling the w/g (if not a regular), as it will allow me to understand if the w/g is legit or here to waste my time.