I am not sure, I went some time ago. You had to purchase online at that time.
Off-Peak Season (September-February) Walk-Up Weekday entry begins at 10 am Monday through Friday. Timed passes will only be required on weekends. Passes are available online three months in advance and are issued the first Wednesday of every month. Same-day online passes are released at 6:30 am, Saturday and Sunday. Groups are required to have passes every day during peak and off-peak seasons. Walk-up entry is not available on Saturdays or Sundays due to high visitation. Peak Season (March-August) Advance Timed Passes and same-day online timed passes are required for entry before 1pm and on the weekends. Walk-Up Weekday entry begins at 1pm Monday through Friday. Passes are available online three months in advance and are issued the first Wednesday of every month. Same-day online passes are released at 6:30 am, Monday – Sunday. Groups are required to have passes every day during peak and off-peak seasons. Walk-up entry is not available on Saturdays or Sundays due to high
Free admission. No tickets required until March or so....the busier season. Cherry blossoms should be blooming in April....plan a DC trip then!
Yes they are free, and they will show you how to get them on your phone.
https://nmaahc.si.edu/visit/passes
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