I would buy at least 2 - 3 days in advance, and yes, you can buy tickets online.
Buy online for the next few days, or go in person the morning of... I bought on the 13th for the opening night. (Dec. 14. 7:30pm)
You can purchase up to 7 days in advance. I recommend purchasing a Regal Ticket Pack from Costco. The tickets end up being $9/each (saving you $4 per ticket on the later showings).
I purchased same day. Movie was packed but I didn't have to wait on line.
If there is an opening you can order in either location.
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