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Gossip Girl‘s 100th episode opens with a Gentlemen Prefer Blondes dream sequence, and we have all the details on Blake Lively‘s Marilyn Monroe dress! “For Serena’s Marilyn look we actually customized Lily’s dress from the pink party in season four,” the show’s stylist Eric Daman told InStyle.com. “We added a giant bow and revamped it a little.” Daman hinted that Serena is wearing her mother’s dress for a special reason, but we’ll have to wait until the episode airs to find out! Catch Serena’s Marilyn moment—and Blair’s royal wedding!—on January 30th at 8/7c on The CW.

Plus! Click through the gallery to see looks from this week’s episode.