Not really...the only days I would say would be Tuesdays or Sundays because Monday is their day off, so if they are going away they would most likely have Monday as one of the days they are away, so they don't have to take a vacation day.
But vacations don't happen that often. Unless an actor is sick they usually are there. So just buy your tickets, and check the board at the theater maybe an hour to an hour and a half before the show, and if the performer you want to see is out, you can always sell your tickets, especially in a show like Wicked where both Eden and Megan are billed above the title. The box office is required to buy back your tickets if a performer who is billed above the title is out.
If you are planning on lottoing or doing cancellation line, try going to the stage door a week or so beforehand if you can and ask the performer you want to see if they have any planned vacations. That way, they will definitely be there, unless they get sick or have some kind of emergency. Hope this helps.
"Free from what, Tracy? Free from what?"