FRIDAY NIGHT OPENING? Really?

What is going on? My routine is being disrupted. Mom just told me that the OPENING of VANYA AND SONIA AND MASHA AND SPIKE by Christopher Durang will be on FRIDAY, April 20th instead of Saturday, April 21st. It seems that one of the cast members (Dakota Graff aka "Nina") has to go to PROM on Saturday night, April 21st! Mom explained how important "prom" is and that in no way were we going to tell her she had to skip "prom". So, we accommodated because that is how we are - sensitive to the needs of others, especially when it comes to "prom". What is a prom?