Madeline tells Rory and Paris that she and Louise have been asked to a party “on the corner of Waverly and First” by Jess and Sean, the boys they picked up at the concert. This is apparently just around the corner from the concert venue.
Waverly Place is in Greenwich Village, not far from New York University, suggesting that Jess and Sean are students there. In real life, there is no point where Waverly Place crosses with First Avenue, although it does cross with Fifth and Sixth. It is very possibly a deliberate error.