Some things that come in groups of seven include the days of the week, the books in the Harry Potter series, the notes in a typically major or minor scale, the games in the playoff series for Major League Baseball and the Pillars of the House of Wisdom in the Book of Proverbs. Other groups of seven include the number of heavens in Islam and the chakras in Hinduism.
Groups of seven found in science include the number of official measurements in the International System of Units, the number of the major stars of Big Dipper constellation, the cervical vertebrae of most mammals, the periods on the periodic table of elements and the seven colors of the rainbow identified by Newton.
Religious groups of seven include the number of blessings at a Jewish wedding, the layers of the Earth in Islam, the Hindu seven worlds in the universe, the Seven Sorrows and the Seven Joys of the Virgin Mary and Jesus' last sayings on the cross. In the Old Testament, there are seven days of creation.
In the classical world, groups of seven include the seven seas, the seven continents, the seven hills of both Rome and Istanbul, and the seven sages of Greece.