nice dress suited for the weather.
when I attended the ballet/opera at ROH, I wore pastel chiffon dresses since it was May/June time.
Winter time--generally more black, add a strand of pearls....:)
yes, "semi formal"--like going to a nice restaurants......all men did wear suits and majority of the women had dresses on.
Awww wow....that must've been wonderful. What did you watch?