Miracle on 34th Street 1947

https://www.imdb.com/title/tt0039628/
Director: George Seaton
Writers: George Seaton / Valentine Davies / Jacques Monteux

At the Macy's Thanksgiving Day Parade, the actor playing Santa Claus is discovered to be drunk by a whiskered old man. Doris Walker, the no nonsense special events director, persuades him to take his place. He proves to be a sensation and is quickly recruited to play Santa at the main store. While he is successful, Doris learns that he calls himself Kris Kringle and he claims to be the actual Santa Claus. Despite reassurances by his doctor that he is harmless, Doris still has misgivings, especially when she has cynically trained herself, and especially her six-year-old daughter, Susan, to reject all notions of belief and fantasy. And yet, people, especially Susan, begin to notice there is something special about Kris and his determination to advance the true spirit of Christmas among the rampant commercialism around him and succeeding in improbable ways. When a raucous conflict with the store's cruelly incompetent therapist, Granville Sawyer, erupts, he finds himself held at Bellevue where, in despair, he deliberately fails a mental examination to ensure his commitment. All seems lost until Doris' neighbor, Fred Gailey, reassures him of his worth and agrees to represent him in the fight to secure his release. To achieve that, Fred arranges a formal hearing in which he argues that Kris is sane because he is in fact Santa Claus. What ensues is a bizarre hearing in which people's beliefs are reexamined and put to the test, but even so, it's going to take a miracle for Kris to win.