History of Advent –
It cannot be determined with any degree of certainty when the celebration of Advent was first introduced into the Church. The preparation for the feast of the Nativity of Our Lord was not held before the feast itself existed. Advent comes from the Latin "Veni" which translates the Greek word parousia - the "coming" of Christ.
One of the earliest references to Christmas being celebrated on December 25 appeared in Antioch in the middle of the second century (100s). At that time, Christians were still persecuted. An official determination was made in the fourth century (300s), when the Roman emperor Constantine embraced Christianity, thereby ensuring the legality of Christmas celebrations. The Council of Tours in 567 established the period of Advent as a time of fasting before Christmas. They also proclaimed the twelve days from Christmas to Epiphany a sacred, festive season.
The time before Christmas hasn't always been a celebration. Advent was once a solemn preparation but not for Christmas. It was a season preparing for Epiphany, January 6—which commemorates the adoration of Jesus by the Magi. Some claim Peter started Advent, but the exact starting even though it’s been lost to history, was probably after Peter’s death.
Whenever it started, Advent originally was a time of fasting and self-reflection - instead of Christmas parties ord "thinking of others for a change." In the 300s, two events changed all that: Emperor Constantine built in Bethlehem, the Church of the Nativity, also declaring Jesus' birthday a holiday; also the Bishop of Rome, Julius, set the date as December 25. Christmas took on a happier atmosphere and became a time of joyous anticipation for the “Next” coming of Christ and commemorating the first one.