Learn German: Der Auftritt ist morgen im Theater. - The performance is in the theater tomorrow.

In German, expressions of time almost always come before expressions of place. This is a core rule of German sentence structure. Because Time comes before Place, the time word morgen must come before the location im Theater.

Why do we use im here instead of in?

im is a short, combined word for in dem. We use the dative case dem here because the sentence describes a fixed location where an event is happening, answering the question 'where?'. Since Theater is a neuter noun (das Theater), it takes dem in the dative.

Can I move morgen to the very beginning of the sentence?

Yes, you can absolutely say Morgen ist der Auftritt im Theater. This is very common if you want to emphasize when the event happens. Just remember that the verb ist must stay in the second position, so the subject der Auftritt swaps places to go right after the verb.

What kind of performance does Auftritt refer to?

der Auftritt usually refers to a live stage performance, like a band playing a gig, a singer, or a comedian's show. It literally comes from stepping onto a stage. For a movie showing at a cinema, you would use a different word like die Vorstellung.