Rockefeller Center was built through the worst years of the Depression, employing about 40,000 people, and John D. Rockefeller Jr. assumed financial responsibility for it after the opera house that was supposed to anchor it pulled out. The photograph of eleven ironworkers eating lunch on a beam 850 feet up was taken here on September 20, 1932, as a publicity shot. Diego Rivera's lobby mural was destroyed in 1934 because he refused to remove Lenin's face. The Christmas tree tradition started in 1931 when workers put up a small balsam on the muddy site and decorated it with tin cans.
Source: Rockefeller Center, official histories of the center, Lunch Atop a Skyscraper, and the Christmas tree tradition; The Museum of Modern Art, Diego Rivera: Murals for The Museum of Modern Art, exhibition materials (2011–2012)