A few days before the movie comes out
It can vary, anywhere from 2 days to 3 weeks or more depending on the movie, the scheduled/expected movies etc. Best bet is to use their app or website and take a look at the date or movie you are looking for.
Usely up to 2 weeks in advance depending when tickets go on sale, and when show times are released. If you can find a showtime on the app or website that usually means you can get a ticket. For big movies (like Endgame and Star wars) you can get over a month in advance.
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