What most Americans would think of somone showing up on Memorial Day (whom they had hired) to do tree work?

If you're planning any kind of a get-together in your yard and don't want it cluttered with tools and debris, then it's up to you to specify to the contractor (that's what he is in this case) not to work on Monday. If he chooses to work on a national holiday, that's up to him. Nothing in the Constitution about thumb screws or pillories for working on national holidays.