Reese Witherspoon celebrated her 33rd birthday with a star-studded party at boyfriend Jake Gyllenhaal's Los Angeles home on Saturday (21Mar09). Guests included Robert Downey Jr. and his wife SUSAN, as well as the actor's sister Maggie Gyllenhaal and her partner Peter Sarsgaard. Witherspoon spent her actual birthday, on Sunday (22Mar09), at the Los Angeles premiere of her new movie MONSTERS VS. Aliens.