"Monday is the mandatory day off for all members of Actors' Equity Association, so you will never find any professional shows open or rehearsing on Mondays, day or evening."
Well if that is true, then were are a LOT of shows breaking the rules, then!
In fact, the above statement is incorrect. As noted by another user earlier, it is required for AEA members to have one day off a week. Most shows tend to choose Monday as the day off because it's the day of the week with the slowest sales. However, other shows choose to perform on Monday to take advantage of the people in the same situation as you -- they're here on a Monday and want to take in a show.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle