Halloween seems to be the starting point of events in the Harry Potter books.
Halloween, 1981: James and Lily Potter are murdered and their son, Harry, is left an orphan. This marks the end of the Wizarding War but the start of Harry Potter - The Boy Who Lived.
Halloween, 1991: Ten years later. Harry Potter is now in Hogwarts. The Halloween feast is interrupted by a Mountain Troll entering the school. Harry Potter and Ron Weasley save Hermione Granger from the Mountain Troll in the girls bathroom. This marks the start of their friendship.
Halloween, 1992: The Chamber of Secrets is opened for the first time in 50 years and Mrs.Norris is petrified by Ginny Weasley who was being controlled by Tom Riddle. This marks the start of a series of attacks that carry on throughout the school year.
Halloween, 1993: Sirius Black enters Hogwarts and attacks the portrait of the Fat Lady. The Gryffindor students have to sleep in the Great Hall while the school is searched for Sirius Black. This marks the start of Sirius being in the castle and him inevitably meeting his Godson and Harry finding out the truth about his parents deaths.
Halloween, 1994: After the Halloween Feast the Three Champions are chosen to compete in the Triwizard Tournament. Harry Potter is chosen as the Fourth Champion. This marks the start of the events leading to Voldemorts rise to power.