Quite a few time. Being in the ACG, we regularly take outings to the Art Gallery and Museum in costume to see exhibitions and many of our events have use wandering about on public transport and at various venues in full costume.
However, a few more regular situations:
Regency Gown for Pride and Prejudice at the Theatre &
Princess Leia for International Star Wars Day at the Cinema
Princess Ariel to see Disney on Ice &
Lolita Mad Hatter for Melbourne Cup Day at work