Freaky Friday is a classic comedic children's novel written by Mary Rodgers, first published in the United States in 1972, and adapted for film several times.
A willful, disorganized teenage girl, Annabel Andrews, awakens one Friday morning to find herself in the body of her mother, with whom she had argued the previous night.
Suddenly in charge of taking care of the New York family's affairs and her younger brother Ben (whom Annabel has not-so-affectionately nicknamed "Ape Face" and said "He's so neat, it's revolting!"), and growing increasingly worried about the disappearance of "Annabel", who appeared to be herself in the morning but has gone missing after leaving the Andrews' home, she enlists the help of her neighbor and childhood friend, Boris, though without telling him about her identity crisis.
As the day wears on and Annabel has a series of increasingly bizarre and frustrating misadventures, she becomes gradually more appreciative of how difficult her mother's life is, and learns, to her surprise, that Ben idolizes her, and Boris is actually named Morris, but has a problem with chronic congestion (at least around Annabel) leading him to nasally pronounce ms and ns as bs and ds. The novel races towards its climax and Ben also disappears, apparently having gone off with a pretty girl whom Boris did not recognize, but Ben appeared to trust without hesitation.
In the climax and dénouement, Annabel becomes overwhelmed by the difficulties of her situation, apparent disappearance of her mother, loss of the children, and the question of how her odd situation came about and when/whether it will be resolved. Finally, it is revealed that Annabel's mother herself caused them to switch bodies through some unspecified means, and the mysterious girl who took Ben was Mrs. Andrews in Annabel's body (to which she is restored) made much more attractive by a makeover Mrs. Andrews gave the body while using it, including the removal of Annabel's braces, an appointment Annabel had forgotten about (and would have missed, had she been the one in her body that day).
The book (and especially the film adaptations and its second sequel, Summer Switch) might be considered a modern retelling of Vice Versa, the 1882 novel by F. Anstey, in which the protagonists are a father and son.