Has anyone been to the museum on a holiday? For example- Labor day? What's the likeliness of getting in as a walk-in?

If you or a family member are military, no ticket is required and you can probably get in. So many people are 'no-shows' that if you are only one or two people, you can probably get in. Go early, let everybody in line know you are looking for tickets.

Wow, I would probably not try it. It was really crowded when I went on a Friday during the summer and no holiday. But sometimes if a group has an extra ticket, u may get lucky.