Okay, I'll be very specific.
A few days before, the outer envelope can be separated from the inner envelope. I will set this period during the election; this is what is normally done. There are two envelopes in a mail-in ballot: an outer envelope with voter information, a bar code and signature, which are security measures, and the inner envelope, which is anonymous. You can start preparing for the separation beforehand, but the actual vote count would probably start on the weekend and continue on Monday. Even if the period is not extended by the change I am proposing, the count would probably take an extra day or two.