Mary Rodgers Guettel, best known as the author of Freaky Friday and playwright behind Once Upon a Mattress (the musical that launched Carol Burnett), passed away on Thursday, June 26, at the age of 83. She was a hugely important member of the New York theater community with Stephen Sondheim even writing the song "Opening Doors" in Merrily We Roll Along as a tribute to the early days of their careers and decades-long friendship. She's survived by her five children and seven grandchildren.