The primary location to meet Anna and Elsa is the Royal Sommerhus in the Norway Pavilion at EPCOT. This beautifully themed summer cabin is their permanent "meet and greet" home, where they typically appear throughout the day to take photos and sign autographs. Unlike other characters who might rotate, the sisters are almost always here together. You can also see them in several other capacities across Walt Disney World: they feature prominently in the "For the First Time in Forever: A Frozen Sing-Along Celebration" at Hollywood Studios (though they don't do individual meets there) and appear on floats during the "Festival of Fantasy Parade" at Magic Kingdom. During the holiday season, they also appear in special shows like "Mickey’s Once Upon a Christmastime Parade." For the most personal interaction, the EPCOT Royal Sommerhus is your best bet, but be prepared for wait times that often exceed 45 minutes.