Advent, anglicized from the Latin word adventus meaning "coming", is a season observed in many Western Christian churches, a time of expectant waiting and preparation for the celebration of the Nativity of Jesus at Christmas. It is the beginning of the Western liturgical year and commences on Advent Sunday, called Levavi. The Eastern churches' equivalent of Advent is called the Nativity Fast, but it differs both in length and observances and does not begin the church year, which starts instead on September 1.

The progression of the season may be marked with an Advent calendar, a practice introduced by German Lutherans. At least in the Roman Catholic, Anglican, Lutheran, Moravian, Presbyterian and Methodist calendars, Advent starts on the fourth Sunday before December 25, the Sunday from November 27 to December 3 inclusive.

Latin adventus is the translation of the Greek word parousia, commonly used in reference to the Second Coming of Christ. For Christians, the season of Advent serves as a reminder both of the original waiting that was done by the Hebrews for the birth of their Messiah as well as the waiting of Christians for Christ's return from Heaven where he now sits at the Right Hand of God.

Otto Olsson (19 December 1879 – 1 September 1964) was a Swedish composer of classical music.

Otto Olsson was one of the greatest organ virtuosos of his time. He studied organ with Lagergren and composition with Dente at the Royal Swedish Academy of Music, and then joined the faculty there, where he taught harmony (1908-24) and then organ (1924-45). He was also the organist at the Gustav Vasa church in Stockholm. He became a member of the Swedish Royal Academy of Music in 1915.

He used his strong background in counterpoint, combined with an affinity for French organ music, to create his late Romantic style of composition. He also had an interest in earlier music and used the plainchant techniques of Gregorian chant in his Gregorianska melodier. He explored polytonality in his work, an advancement not found in other Swedish works of the time. In addition to many fine works for the organ, instrumental and choral works, his best-known work is his setting of the Te Deum, a large piece for chorus, string orchestra, harp, and organ.