What is Advent?

Legal Definition
Advent is a season observed in many Western Christian churches as a time of expectant waiting and preparation for the celebration of the Nativity of Jesus at Christmas. The term is a version of the Latin word meaning "coming".

Latin adventus is the translation of the Greek word parousia, commonly used to refer to the Second Coming of Christ. For Christians, the season of Advent anticipates the coming of Christ from three different perspectives. "Since the time of Bernard of Clairvaux (d.1153) Christians have spoken of the three comings of Christ: in the flesh in Bethlehem, in our hearts daily, and in glory at the end of time." The season offers the opportunity to share in the ancient longing for the coming of the Messiah, and to be alert for his Second Coming.

Advent is the beginning of the Western liturgical year and commences on the fourth Sunday before Christmas (sometimes known as Advent Sunday), the Sunday nearest to St. Andrew's Day (30 November), in the Roman Rite of the Catholic Church, and in the Anglican, Lutheran, Moravian, Presbyterian and Methodist calendars. In the Ambrosian Rite and the Mozarabic Rite of the Catholic Church, Advent begins on the sixth Sunday before Christmas, the Sunday after St. Martin's Day (11 November).

Practices associated with Advent include keeping an Advent calendar, lighting an Advent wreath, praying an Advent daily devotional, lighting a Christingle, as well as other ways of preparing for Christmas, such as setting up Christmas decorations, a custom that is sometimes done liturgically, through a hanging of the greens ceremony.

The equivalent of Advent in Eastern Christianity is called the Nativity Fast, but it differs in length and observances, and does not begin the liturgical church year as it does in the West. The Eastern Nativity Fast does not use the equivalent parousia in its preparatory services.
-- Wikipedia
Legal Definition
A period of time recognised by the English common and ecclesiastical law, beginning on the Sunday that falls either upon St. Andrew's day, being the 30th of November or the next to it, and continuing to Ohrlstmas day. Wharton.
-- Black's Law Dictionary
Legal Definition
The period from the Sunday nearest November 30th until Christmas.
-- Ballentine's Law Dictionary