The Advent season begins on November 28, 2021. The day is called the First Sunday of Advent or Advent Sunday and is the first day of the liturgical year in the Western Christian churches. Advent Sunday is the fourth Sunday before Christmas Day. This is equivalent to the Sunday nearest to St. Andrew's Day and the Sunday following the Feast of Christ the King. It can fall on any date between 27 November and 3 December.
The theme of readings and teachings during Advent is often to prepare for the Second Coming while commemorating the First Coming of Christ at Christmas. With the view of directing the thoughts of Christians to the first coming of Jesus Christ as savior and to his second coming as judge, special readings are prescribed for each of the four Sundays in Advent.
In recent times the most common observance of Advent outside church circles has been the keeping of an advent calendar or advent candle, with one door being opened in the calendar, or one section of the candle being burned, on each day in December leading up to Christmas Eve. The keeping of an advent wreath is also a common practice, with four or five candles extending from the wreath. The colours associated with the candles are variously interpreted.
The usual liturgical colour in Western Christianity for Advent is either purple or blue. The purple color is often used for hangings around the church, on the vestments of the clergy, and often also the tabernacle. (With material from: Wikipedia)