Learn Russian: Сегодня в театре аншлаг. - It's a full house at the theater today.

It is a very common theater term meaning a full house or a sold-out performance. It originally comes from the German word for a notice or poster. In Russian theater history, a Sold Out sign would be pasted over the playbill, and eventually, the word came to mean the sold-out state itself.

Why is аншлаг placed at the very end of this sentence?

Russian sentence structure flexibly places the most important new information at the very end. By starting with the time (сегодня) and the location (в театре), the sentence sets the scene first, and then delivers the main point: the fact that it is a full house (аншлаг).

How would this sentence change if I were talking about a past performance?

You would need to add the past tense of the verb to be. Because аншлаг is a masculine noun, you would use был, making the sentence Сегодня в театре был аншлаг. In the present tense, this verb is simply omitted.

If I wanted to emphasize this full house using this lesson's grammar (ни один), how would I say that there isn't a single empty seat?

You would say Нет ни одного свободного места. Because this is a negative sentence stating that something does not exist, both the emphatic pronoun ни один and the noun phrase свободное место (empty seat) must take the genitive case.