Seven

"...isn't seven the most powerfully magical number, wouldn't seven-?"

- Tom Riddle discusses the power of the number seven

Seven is the most powerful magical number, based on centuries of mythology, science, and mathematics.

Wizarding community
The Arithmancer Bridget Wenlock (1202–1285) was the first witch to establish the magical properties of the number seven.

Seven is the age by which most experts believe that magic will reveal itself in a wizard or witch. In the life of the Hogwarts School of Witchcraft and Wizardry community, it is the number of years students must attend Hogwarts before graduating. There were seven chambers protecting the Philosopher's Stone. Lord Voldemort was such a believer in the magical properties of the number seven that he intended to split his soul into seven portions by creating six Horcruxes. Ironically, he instead succeeded in creating seven Horcruxes, thus splitting his soul eight ways - however, by pure chance, only six of his Horcruxes ever existed at one time (as the Diary was destroyed before Nagini became a Horcrux). Harry Potter and Neville Longbottom were born in July, the seventh month of the year; this made them both potential subjects of a prophecy. Arthur and Molly Weasley had seven children; Ginny Weasley is the seventh of them, and the only daughter. Also Ginny Weasley is the first girl to be born to the name Weasley in seven generations. There are also seven players on a standard Quidditch team, with seven also being the number on Harry Potter's Quidditch uniform. During the Battle over Little Whinging, six others took Polyjuice Potion in order to take on Harry's appearance as a diversion, thus there were seven Potters.

Seven seen in the wizarding world

 * S.P.T to A.P.W.B.D: "The one with the power to vanquish the Dark Lord approaches ... born to those who have thrice defied him, born as the  seventh  month dies ... and the Dark Lord will mark him as his equal......"
 * Molly and Arthur Weasley had seven children.
 * When Dumbledore first goes to Privet Drive, he puts out seven lights with his deluminator.
 * Weasley contains seven letters.
 * "Dragon liver, seven teen Sickles an ounce, they're mad..." - Diagon Alley witch
 * Ginny Weasley is the first girl born to the Weasleys in seven generations and likewise she is the seventh child of a seventh daughter. Both of these are known to have contributed to her exceptional magical talent.
 * Thaddeus Thurkell had seven children, all Squibs.
 * Walter Parkin has seven children.
 * Before Harry Potter and Ronald Weasley came of age, seven people owned the pocket watch.
 * Children go to school at Hogwarts for seven years.
 * There are seven floors at Hogwarts.
 * In the wizarding world, wizards and witches come of age at the age of seventeen, instead of eighteen.
 * Harry Potter paid seven Galleons for his wand.
 * There are seven hundred fouls in Quidditch.
 * There are seven players on a Quidditch team
 * The wolfsbane potion must be drunk every seven days before full moon.
 * The found index finger of Peter Pettigrew is the seventh finger if you count from one to 10.
 * Seven people come out of the Shrieking Shack in 1994 after Peter Pettigrew's crimes are uncovered:
 * Harry Potter
 * Ronald Weasley
 * Hermione Granger
 * Remus Lupin
 * Sirius Black
 * Severus Snape
 * Peter Pettigrew


 * There are seven obstacles leading up to the Philosopher's Stone:
 * Hagrid's pet, Fluffy
 * Sprout's Devil's Snare
 * Flitwick's flying keys
 * McGonagall's giant chess set
 * Quirrell's troll
 * Snape's Potions
 * The Mirror of Erised


 * Gilderoy Lockhart assigned seven books for his students:
 * Break with a Banshee
 * Gadding with Ghouls
 * Holidays with Hags
 * Travels with Trolls
 * Voyages with Vampires
 * Wanderings with Werewolves
 * Year with the Yeti


 * Seven Ghosts of Hogwarts are mentioned more than once:
 * Nearly Headless Nick
 * The Grey Lady
 * The Fat Friar
 * The Bloody Baron
 * Peeves the Poltergeist
 * Professor Binns
 * Moaning Myrtle


 * In Philosopher's Stone, Hermione says that the spell 'Alohomora' was in chapter 7 of The Standard Book of Spells.
 * The Philosopher's Stone was kept in Vault 713 in Gringotts Bank.
 * Seven entities were attacked by the Basilisk during the 1992–1993 school year:
 * Mrs. Norris
 * Colin Creevey
 * Justin Finch-Fletchley
 * Nicholas de Mimsy-Porpington
 * Hermione Granger
 * Penelope Clearwater
 * Harry Potter


 * There are seven snakes on the door of the Chamber of Secrets.
 * There are seven locks on Professor Moody's trunk
 * Broomstick named Cleansweep Seven
 * Seven bottles in the last task in the Philosopher's Stone chambers
 * Seven Muggles saw Ron and Harry in the Flying Ford Anglia
 * Seven keepers for every dragon for the first task of the Triwizard Tournament
 * Mr. Weasley won seven hundred galleons in the Daily Prophet drawing
 * Harry helped Gryffindor beat Slytherin for the House Cup for the first time in seven years
 * Only seven people knew where the entrance to the Chamber of Secrets was before the basilisk within was killed.
 * Salazar Slytherin
 * Tom Riddle
 * Myrtle
 * Harry Potter
 * Ron Weasley
 * Ginny Weasley
 * Gilderoy Lockhart


 * There are seven registered Animagi of the 20th century:
 * Minerva McGonagall — Tabby cat
 * Unidentified witch — Cat
 * Five unknown animagi — Forms unknown

(Note that Harry was in the hospital wing for the Gryffindor vs. Ravenclaw match in his first year. Quidditch was cancelled before Gryffindor could play Hufflepuff or Ravenclaw due to the basilisk attacks in Harry's second year. Harry fell off his broom due to because of dementors and Cedric Diggory got the Snitch in Harry's third year match of Gryffindor vs. Hufflepuff. There were no Hogwarts Quidditch matches in Harry's fourth year because Hogwarts was hosting the Triwizard Tournament. Harry earned a "lifelong" ban from Quidditch after beating up Draco Malfoy, and therefore, Ginny Weasley played Seeker in the Gryffindor matches against Hufflepuff and Ravenclaw. During Gryffindor's match against Hufflepuff in Harry's sixth year, the reserve Keeper, Cormac McLaggen, knocked Harry unconscious with a misaimed Bludger before Harry could get the Snitch, and Harry was put in detention by Professor Snape during the final match against Ravenclaw. Harry did not return to Hogwarts for his seventh year, and therefore could not play Seeker.)
 * The Marauder's Map shows seven secret passages out of the school (the map does not show the Room of Requirement, and therefore nor its passage to Hogsmeade).
 * From the Great Hall, Harry, Ron, and Hermione climb seven staircases to reach the tower that houses Professor Trelawney's classroom at the top.
 * Out of the nine members of the Order of the Phoenix in the Advance Guard Harry hadn't already been acquainted with seven of them beforehand.
 * The mother of Blaise Zabini had been married seven times as of 1996.
 * In 1996, while brewing the Draught of Living Death in a Potions class, Harry followed the alternate instructions in the Half-Blood Prince's copy of Advanced Potion-Making, by stirring his potion once clockwise after every seventh stir counter-clockwise. This helped him achieve much better results than the standard instructions, and earn Slughorn's praise.
 * Harry caught seven Golden Snitches during his time as Seeker on the Gryffindor Quidditch team:
 * 1) First year: Gryffindor vs. Slytherin
 * 2) First year: Gryffindor vs. Hufflepuff
 * 3) Second year: Gryffindor vs. Slytherin
 * 4) Third year: Gryffindor vs. Ravenclaw
 * 5) Third year: Gryffindor vs. Slytherin
 * 6) Fifth year: Gryffindor vs. Slytherin
 * 7) Sixth year: Gryffindor vs. Slytherin
 * Dumbledore shows Harry seven memories about Voldemort's past (including both the fake and real Horcrux memory).
 * Marvolo, Tom Riddle's middle name, has seven letters in it.
 * There are (possibly) seven Standard Book of Spells.
 * Voldemort made seven Horcruxes, even though he believed that six Horcruxes would create a seven-fragment soul (including the piece of soul in his body).
 * Tom Riddle's Diary
 * Marvolo Gaunt's Ring
 * Rowena Ravenclaw's Diadem
 * Salazar Slytherin's Locket
 * Helga Hufflepuff's Cup
 * Harry Potter (unintentionally and unknown to Voldemort)
 * Nagini


 * There are seven "Potters" during the Battle over Little Whinging:
 * Harry Potter
 * Ron Weasley
 * Hermione Granger
 * Fred Weasley
 * George Weasley
 * Fleur Delacour
 * Mundungus Fletcher


 * There are also seven conflicts in Harry Potter and the Deathly Hallows:
 * The Battle over Little Whinging
 * Infiltration of the Ministry of Magic
 * The Attack at Godric's Hollow (1997)
 * Ambush at the Lovegood House
 * The Battle of Malfoy Manor
 * 1998 Break-in of Gringotts Wizarding Bank
 * The Battle of Hogwarts


 * Seven prisoners escape from Malfoy Manor:
 * Mr. Ollivander
 * Luna Lovegood
 * Dean Thomas
 * Griphook
 * Harry Potter
 * Hermione Granger
 * Ronald Weasley


 * Harry Potter escaped death at the hands of Lord Voldemort seven times before finishing him off:
 * In Godric's Hollow in 1981
 * Over the Philosopher's Stone while Voldemort was in control of Quirinus Quirrell
 * In the Chamber of Secrets in 1993
 * In Little Hangleton in 1995
 * During the Battle of the Department of Mysteries
 * During the Battle over Little Whinging
 * During the Battle of Hogwarts


 * Harry Potter and Ron Weasley received seven O.W.Ls for their Ordinary Wizarding Level examinations. Hermione Granger continued with seven subjects during her sixth year.
 * Sybill Trelawney says that Seven of Spades means an ill omen.
 * When a wizard is seven, they start showing magical properties.

Behind the scenes

 * There are seven books in the Harry Potter series, and eight movies.
 * The first years enter the castle in the seventh chapter of the first book.
 * Many fans expected Harry Potter and the Deathly Hallows (the seventh book) to be released on 07/07/2007, which also happened to be a Saturday, the day upon which Harry Potter books had previously been released. However, it was released on July 21st of that year instead, two weeks later. It is thought that the more 'magical' date was rejected due to it also being the second anniversary of the 7/7 London Bombings in 2005. (21 is still a multiple of 7 though.)
 * The number on the back of Harry Potter's Quidditch robes in the films is seven (this could be a reference to the fact that usually the best player/captain in a football team, has the number 7 shirt).
 * Chapter four of Harry Potter and the Deathly Hallows is entitled "The Seven Potters".
 * In the films, Voldemort is played by seven actors: Richard Bremmer, Ian Hart, Christian Coulson, Ralph Fiennes, Hero Fiennes-Tiffin, Frank Dillane and by Michael Berendt althrough the latters scenes were cut and never seen.
 * J. K. Rowling stated in interview that she chose to write seven books in total because she spent seven years in secondary school. Also, she wanted Harry to come of age at seventeen because it "just seems a good number for a wizard to come of age."
 * In the seventh Harry Potter book, J.K. split the dedication in seven parts.
 * J. K. Rowling stated in interview that the "big seven" in the Harry Potter series are: Harry Potter (Daniel Radcliffe), Hermione Granger (Emma Watson), Ron Weasley (Rupert Grint), Ginny Weasley (Bonnie Wright), Neville Longbottom (Matt Lewis), Luna Lovegood (Evanna Lynch), and Draco Malfoy (Tom Felton).
 * There were seven clues and seven opportunities to sign up and register for the Pottermore beta. The first clue was on 31 July, 2011 (Harry's birthday) and the final was 6 August, 2011. A clue was released each day and each clue was from a book in the series corresponding to the day. For example, day one's clue was from Philosopher's Stone and the final clue was from Deathly Hallows.
 * On 7 August, the Harry Potter series won seven awards at the 2011 Teen Choice Awards for Deathly Hallows: Part 1 and Part 2.