VIDEO : Taylor Swift & Idina Menzel Sing 'Let It Go' in Frozen Costumes

How's this for a Taylor Swift Halloween surprise? The pop star was certainly in the holiday spirit at her concert at Raymond James Stadium in Tampa, Florida on Saturday night, as she donned a costume herself and brought out a special celebrity guest who was dressed perfectly for the magical moment. It was Idina Menzel , who not only sang with her  that song, yes, "Let It Go," from Disney's Frozen , but also looked absolutely stunning dressed in her character Elsa's sparking turquoise gown and aqua cape.