That's a great idea to announce the date even if they don't have the lineup yet. At least people can plan it around their vacation days, or book ahead their hotel etc...
I went last year for the first time and it was a very fun/relaxed experience. it's a great opportunity to hear artist sing songs from shows they were on in the past, cover some pop songs as well as hear some interesting collaborations between different artists. A great effort to bring something new for musical theatre!