Pope Pius XI promulgated the encyclical Quas primas, establishing the Feast of Christ the King.

Pope Pius XI was head of the Catholic Church from 6 February 1922 until his death in February 1939. He was also the first sovereign of the Vatican City State upon its creation on 11 February 1929.

An encyclical was originally a circular letter sent to all the churches of a particular area in the ancient Roman Church. At that time, the word could be used for a letter sent out by any bishop. The word comes from the Late Latin encyclios. The term is now primarily associated with papal encyclicals.

Quas primas is an encyclical of Pope Pius XI. Promulgated on December 11, 1925, it introduced the Feast of Christ the King.

The Solemnity of Our Lord Jesus Christ, King of the Universe, commonly referred to as the Feast of Christ the King, Christ the King Sunday or Reign of Christ Sunday, is a feast in the liturgical year which emphasises the true kingship of Christ. The feast is a relatively recent addition to the liturgical calendar, instituted in 1925 by Pope Pius XI for the Roman Rite of the Catholic Church. In 1970, its Roman Rite observance was moved from October to the last Sunday of Ordinary Time and thus to the end of the liturgical year. The earliest date on which the Feast of Christ the King can occur is 20 November and the latest is 26 November. It typically marks the end of Ordinary Time, which continues up until Advent Sunday, the first day of Advent. Depending on the year, Saint Andrew's Day, significant in some cultures, may fall prior to Advent Sunday. Thus, Christ the King is typically the last or second-to-last calendrical feast of the liturgical year.