Ashley Olsen and Mary Kate Olsen
Credit: Dave Allocca/Startraks

Ashley and Mary-Kate Olsen joined entertainment industry "It" girls Anna Kendrick, Zoe Kazan and Zoe Kravitz for the premiere of The Union on opening night of the 2011 Tribeca Film Festivalin New York City. To mark the 10th anniversary of the downtown movie fest, Sir Elton John treated the crowd to a post-screening performance. In Los Angeles, Baby2Baby ambassadors Nicole Richie and Jessica Alba showed their support for moms in need, while Scarlett Johansson and Jennifer Garner shopped for a cause at Coach's benefit for the Children's Defense Fund. See photos from last night's hottest parties in the gallery!