What is Advent?

Advent is a season of the liturgical year observed in many Christian churches as a time of expectant waiting and preparation for both the celebration of the Nativity of Jesus at Christmas and the return of Jesus at the Second Coming. Advent is the beginning of the Western liturgical year. The term "Advent" is also used in Eastern Orthodoxy for the 40-day Nativity Fast, which has practices different from those in the West.

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https://en.wikipedia.org/wiki/Advent#Liturgical_colours

What is Advent?