Rescission usually includes Saturday because it doesn't matter that you are opened or closed, the consumer can mail you the notice.
I don't know that it was designed this way, I assume so. But I have always counted the days when the US Mail was running. That means Sundays and Federal holidays are out. All others are in.
You might check your state's laws for any quirky requirement pertaining to Saturdays, state holidays, etc.
Opinions stated are not necessarily that of my employer.