"Do you know when the dance is?"
Translation:¿Sabes cuándo es el baile?
Duolingo is using 'es' from the verb ser with times such as when a movie, game etc starts. It's using "está' from the verb estar for locations. That's the correct use from what I get from the language guide on the use of ser and estar on SpanishDict!.com. https://www.spanishdict.com/guide/ser-vs-estar
my understanding is when something that is fixed and constant, we use SER. when something that is subject to change, we use ESTAR. however, location etc, we use estar, even it does not change often. Here, as you mentioned, the time for start of game, movie etc, it is not changing whence the time is determined, but in a lot of situations, it is not always clear which of the two words should be used. If a mistake were made, I do not feel it is big deal when we are talking in every day life, as long as another party understands. DL is just teaching the right way as much as it can.