Advent (from the Latin word adventus, meaning "coming") is a season of the Christian church, the period of expectant waiting and preparation for the celebration of the Nativity of Jesus, in other words, the period immediately before Christmas. It is the beginning of the Western Christian year and commences on Advent Sunday.
Latin adventus is the translation of the Greek word parousia, commonly used in reference to the Second Coming. Christians believe that the season of Advent serves a dual reminder of the original waiting that was done by the Hebrews for the birth of their Messiah as well as the waiting that Christians today endure as they await the second coming of Christ. (From Wikipedia)
O Come, O Come Emmanuel
O come, O come, Emmanuel,
And ransom captive Israel,
That mourns in lonely exile here
Until the Son of God appear.
O come, Thou Day-spring, come and cheer
Our spirits by Thine advent here;
Disperse the gloomy clouds of night,
And death’s dark shadows put to flight.
"Therefore keep watch, because you do not know the day or the hour." Matthew 25:13