Mondays during previews are quite common as they allow for an additional afternoon of rehearsal time.
That they're switching back to Mon-Sat schedule in the summer is interesting. It actually used to be more common -- during the summer, when so many locals are away on the weekends, producers felt that Sundays were weak and a Monday night performance might do better than a Sunday matinee. They would then switch back to the more common Tuesday-Sunday schedule after Labor Day. On the flip side, there are those who feel that summer Sundays are still the better bet for the tourist audience, and playing Monday weakens Tuesday night, which in turn further weakens Wednesday night, which is already typically the worst show of the week for most productions. There's no right answer here -- each show is different so I give them credit for experimenting.
Either way, it looks like they're only putting ticket on sale through Labor Day so will have to wait and see if they switch back or not.
"No matter how much you want the part, never let 'em see you sweat." -- Old Dry Idea commercial