I purchased on the day. Early in the morning
If you are in the city on a Saturday, you can request tickets by clicking on the same day tickets link on their website at 6:30am. Once you receive the e-tickets it will tell you what time you can arrive.
The museum is free to the public. It's easier to visit during the week then the weekend. Depending on how large the group is you would need to schedule a visit. Link below. Hope this helps. http://event.etix.com/ticket/e/1004039/timed-entry-pass-washington-national-museum-of-african-american-history-and-culture
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