Seven

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

Wizarding Community
The Arithmancer Bridget Wenlock, who lived from the year 1202–1285, was the first witch to establish the magical properties of the number seven.

In the life of the Hogwarts community, it is the number of years students must attend Hogwarts before graduating. It is the number of Horcruxes created by Lord Voldemort, specifically because he believed in the numerological power of the number. Harry Potter and Neville Longbottom were born in July, the seventh month of the year; this made them both potential subjects of a prophecy. Ginny Weasley is the seventh Weasley child, and only daughter. There are also seven players on a standard Quidditch team. 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".

Science and Nature
Seven is a common number in science and nature. It is the number of spots on a common ladybug. All mammals' necks have seven bones, with very few exceptions. There are seven colours in the visible light spectrum: Red, orange, yellow, green, blue, indigo, and violet. September was the seventh month of the calendar before July and August were added. There are also seven seas on planet Earth.

The Native Americans of Montana have seven seasons: chinook season, muddy spring, green summer, gold summer, 'Indian' summer, late fall, and cold winter.

There are seven heavenly bodies visible to the naked eye: Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn. In addition, there are seven stars in the star cluster Pleiades (also called the "seven sisters"), which some members of the House of Black and Gaunt are named after.

There are traditionally "seven wonders" of the ancient world. There were seven emperors of the classical age of Rome. In the English history period of 500-850 AD, there are seven kingdoms that would eventually become the Kingdom of England in the 10th century: Northumbria, Mercia, East Anglia, Essex, Kent, Sussex, and Wessex.

A common winner combination of most slot machines is 7-7-7.

Magic and Mysticism
The number seven holds great power in the realms of magic and mysticism. In Judaism, the number seven alludes to the infusion of spirituality and Godliness. For example, traditionally it is held that "God rested on and sanctified the seventh day", or Sabbath. Seven blessings are recited under the chuppah during a Jewish wedding ceremony (Sheva Berachot). Moreover, there are the "Seven Shepherds" who visit the sukkah during the holiday of Sukkot: Abraham, Isaac, Jacob, Joseph, Moses, Aaron, and David. There were seven nations promised to the Israelites when they entered the land of Israel. The menorah holds seven candles.

Seven further has a presence in mysticism as Yahweh or Jehovah is the divine name in Christianity, whose letters alphabetically equal 70, a product of the divine number 7. There are traditionally seven virtues: Chastity, Moderation, Liberality, Charity, Meekness, Zeal, and Humility. Correspondingly, there are seven deadly sins and seven holy sacraments. According to some traditions, there are seven Archangels.

In Hindu tradition, there are seven worlds in the universe and seven chakras. In Japanese mythology, there are seven Lucky Gods. In the British folk tale Tam Lin, every 7 years the Fairy Queen pays a tithe to Hell. Moreover, in the British tale of of Thomas the Rhymer, he went to live in the faerie kingdom for 7 years. There are seven basic principles of Bushido.

There were also seven main islands constituting the mythological land of Atlantis.

Behind the scenes

 * Many fans expected Harry Potter and the Deathly Hallows (the seventh book) to be released on 07/07/2007, but it was released on July 21st of that year instead.
 * The number on the back of Harry Potter's Quidditch robes in the Prisoner of Azkaban movie is seven.
 * Chapter four of Harry Potter and the Deathly Hallows is entitled "The Seven Potters".
 * 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."