muscle23ftl is right! Just be your own self! Dress comfortably, if not elegantly (well, we ALWAYS try to dress with some elegance!!).
Nothing really happens for an opening matinee. The Press event is most likely the true opening and a private invite (and if it's the matinee, I'd be surprised!?!). Sometimes shows just stop in to towns with little fanfare.
If you're going backstage or a party afterwards, then you should dress appropriately.
Either way, have fun! Great show...
"It's not so much do what you like, as it is that you like what you do." SS
"Time flies like an arrow, fruit flies like a banana." GMarx