Time Out says
In the spirit of Smithsonian Museums, which offer free admission every day, Museum Day is an annual event hosted by Smithsonian magazine in which participating museums across the country open their doors to anyone presenting a Museum Day ticket… for free. Frequent participating museums include the Japanese American National Museum, GRAMMY Museum and Museum of Latin American Art, as well as some smaller oddities within a few hours of L.A.
Want more free museums, more of the time? These L.A. museums offer free admission every single day.