7 truly is a magical number.

7 books in the series
7 horcuxes
7 years at Hogwarts
7 floors of Hogwarts
7 galleons for a wand
7 positions in quidditch
7 tasks in the Philosopher's/Sorcerers Stone
7 potions in task 6
7 Weasley children
Ginny is the 1st Weasley girl in 7 generations.
Gryffindor beats Slytherin for the house cup for the first time in 7 years in the Philosopher's/Sorcerers Stone
7 books Gilderoy Lockhart requires for DADA (CoS)
7 muggles see Harry and Ron fly the car (CoS)
7 days of Aunt Marge (PoA)
Arthur Weasley wins 700 galleons (PoA).
7 tear drops on Hagrid’s letter to Hermione (PoA).
7 people in the Shrieking Shack (PoA): Harry, Hermione, Ron, Sirius, Lupin, Snape, and Pettigrew.
Voldemort killed Frank Bryce who was 77 to make the 7th horcrux.
7 locks on Moody’s trunk (GoF)
Dobby has 7 socks (GoF)
Unicorns don’t turn pure white until they’re 7 years old. (GoF)
Harry was “born as the 7th month dies…” (OotP)
7 memories of Tom Riddle (HBP)
Harry and Ron get 7 O.W.L.s each (HBP)
7 DADA teachers
7 questions Bellatrix asks Severus in Spinner’s End
7 Harry Potters with 7 Order members
7 races in the wizarding world: Human, Giant, Goblin, Centaur, Elf, Werewolf, Veela
There are 142 stairs at Hogwarts which adds to 7 (1+4+2=7)
Cleansweep 7
Gryffindor Tower is located on the 7th floor
Nicholas Flamel and his wife have 7 years age difference
7 hidden passageways out of Hogwarts on the Marauders’ Map
Flitwick’s office (where Sirius is locked in PoA) is on the 7th floor
700 ways to commit a foul in Quidditch.
The Tri-Wizard Tournament was first established 700 years before it’s appearance in the GoF.
Fred and George charge 7 sickles for a canary cream
Clause Seven of the Decree states that magic may be used before Muggles in exceptional circumstances
The Room of Requirement, used for DA meetings, is on the 7th floor. (OotP)
Cormac McLaggen’s mother was married 7 times. (HBP)
7 death eaters at the tower in HBP: Draco, Fenrir, Amycus, Alecto, tall blond, Snape, Gibbon as well as 7 members of the Order and the DA: McGonagall, Tonks, Lupin, Neville, Ginny, Hermione, Ron
Lily began going out with James in their 7th year at Hogwarts
The prophecy is in row 97 in the Department of Mysteries
There are 7 Animagi registered with the Improper Use of Magic Office
7 people locked in the Malfoy’s cellar (DH): Ollivander, Luna, Dean, Harry, Dean, Ron, and Griphook