If you use a website like fandango or ticketmaster it shouldn't be a problem.
Fandango worked for me
Use fandango or atom apps on your phone and you can purchase tix in advance
The answer to this is movie specific, but I did look up Star Wars and there are not any current showtimes for either 12/31 or 1/1. Usually they are posted by now, but I would check back Wednesday afternoon.
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