Yom Kippur is September 18th and Rosh Hashanah is September 28th. Many non-church affiliated private schools and universities (i.e. University of Southern California) in Los Angeles are closed for one or both of these holidays (usually Yom Kippur when its only one of them). Additionally, at schools that don't close, students are not penalized for missing either day.
I have no idea what effect these two have on crowds, since many people who observe the holiday go to temple, but many others (i.e. me) take it as a family day.