As I said, I expect, so I'm not 100% certain.
If you look back over the entire season, the presale for members 80-90% of the time started at 10hrs00 CET. Exactly which day of the week depended on various factors. The other 10-20% of the time, presale started at 16hrs00. Those 2 timeslots are always recurring.
Since both 10hrs00 CET & 16hrs00 CET have passed now, and sales haven't started, I assume they'll go for the next earliest option: tomorrow morning.
If the past is anything to go by: tomorrow & Wednesday will be the sale for members. Wednesday evening Juventus will communicate whether or not there are still tickets available to start a general sale. If there are, it will only be a few 100 imo.