Best times to go to the museum

When are these NY museums the least crowded?

Metropolitan Museum of Art: Wednesday mornings 9:30am, right after opening.
 http://www.metmuseum.org/

Hours:
Tuesday–Thursday: 9:30 a.m.–5:30 p.m.
Friday and Saturday: 9:30 a.m.–9:00 p.m.
Sunday: 9:30 a.m.–5:30 p.m.
Closed Monday (except Holiday Mondays), Thanksgiving Day, and December 25


American Museum of Natural History:  Sunday mornings before noon and Tuesdays from 1 to 4pm.

Museum of Modern Art :  Wednesdays after 3:30pm.
Whitney Museum of American Art:   Thursdays and Fridays between 4 and 6pm.
Guggenheim:  Wednesdayafternoons after 3pm.
Rubin:  Not sure yet.