MONDAY, AUGUST 31

"GOSSIP GIRL" � (9:00-10:00 p.m. ET)

"Valley Girls" (TV-14, DL) (HDTV)

EPISODE FEATURES GUEST STARS BRITTANY SNOW ("AMERICAN DREAMS"), KRYSTEN RITTER ("CONFESSIONS OF A SHOPAHOLIC"), 80S ICON ANDREW MCCARTHY, MICHELLE TRACHTENBERG ("17 AGAIN") AND A PERFORMANCE BY NO DOUBT � In a flashback to the 80s, 17-year-old Lily Rhodes (Snow) gets kicked out of boarding school and runs away to Los Angeles, reconnecting with her parents, Rick (McCarthy) and Cece (guest star Cynthia Watros, "Lost"), and sister, Carol (Ritter). Back in present day, Blair (Leighton Meester) and Nate (Chace Crawford) attend prom together, but the evening doesn't turn out exactly as planned and Chuck (Ed Westwick) has a little something to do with that. Serena (Blake Lively) and Lily (Kelly Rutherford) don't see eye-to-eye over Serena's recent run-in with the law. No Doubt performs in the episode as an 80s band called "Snowed Out." Penn Badgley, Taylor Momsen, Jessica Szohr and Matthew Settle also star. Marc Piznarski directed the episode written by Josh Schwartz & Stephanie Savage (#224).