21 museums across San Francisco will be open to the public free of charge on the weekend of December 3, thanks to some generous, anonymous patrons of the arts who are underwriting the weekend.

The Chronicle reported Monday that the SF Free Museum Weekend will be divided into two days, December 3 and 4, with a group of museums open for free each day. These include museums both big and small — and the special exhibitions, like the Diego Rivera retrospective at SFMOMA, will be free as well.

"In these economically challenging and politically divided times, museums provide a critical place of discourse, reflection and inspiration,"says Thomas P. Campbell, director and CEO of the Fine Arts Museums of San Francisco, in a statement. “Increasing access to art spaces and removing barriers to experiencing art is crucial.”

The museums are asking for people to reserve tickets — and these are likely to be crowded days.

Saturday, December 3 will be the day for the de Young Museum and Legion of Honor, the Cartoon Art Museum; Museum of Craft and Design, the Walt Disney Family Museum, California Academy of Sciences, GLBT Historical Society Museum, American Bookbinders Museum: Letterform Archive, SF Camerawork, McEvoy Foundation for the Arts: Truhlsen-Marmor Museum of the Eye, Institute of Contemporary Art San Francisco, and the Minnesota Street Project.

And on Sunday, December 4, the free museums will be SFMOMA, the Contemporary Jewish Museum, the Museum of African Diaspora, Yerba Buena Center for the Arts, Children’s Creativity Museum, the Asian Art Museum, and The Exploratorium.

Some museums across the city offer free days once a month — like SFMOMA, which for many years has had a tradition of being open for free on the first Thursday of every month. But this is the first time in memory that such a coordinated free museum day has been organized around the holidays.

"The San Francisco Free Museum Weekend couldn’t come at a better time," says Jay Xu, director and CEO of the Asian Art Museum, in a statement. "As we re-emerge from the challenges of the past months, Bay Area museums are here to help our communities reconnect with each other and with the artists that inspire them."

Photo: Georg Eiermann