Just go online and get your tickets. But the tickets go fast. If you are military, retired or active, you dont need tickets.
Unfortunately advanced tickets for October have already been claimed. You can obtain same day tickets here: https://nmaahc.si.edu/visit/passes . You must login at 630a to claim the timed tickets. They go very fast so you must secure them when you see them. They also do limited walk up tickets starting at 1p on weekdays.
From what I heard, they are taking reservations for Feb. Go on line and make reservations. No charges.
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