Here is a short explanation regarding the historical origins of Advent:
1) The LORD lead the People of God to commemorate certain saving events in a sort of Jewish liturgical year. The annual celebration of Passover is a great example. Every year they remember, celebrate and commemorate with sacred ceremonies their liberation from slavery in Egypt, the Exodus.
2) In the earliest days of the Church, Christianity was but a small sect among many sects in the Jewish faith of the time. By the end of the first century, Christianity had separated from the synagogue and was considered a separate religion.
3) You can read about the first great Christian controversy in Acts 15 and Galatians. Guided by the Holy Spirit, the leaders of the early Church decided that non-Jewish converts did not have to accept certain Jewish practices.
4) As Christianity spread, grew and developed the early Church started to imitate Jewish liturgical sensibilities. Before the year 100 AD, Christians were celebrating Easter with some sort of time of preparation (which became what we now call Lent). By the year 200 AD, some Christians were celebrating the Birth of Christ and some sort of time of preparation (which became what we now call Advent).
5) Before the year 400 AD, we find references in several writings bearing witness to various practices of preparation for the celebration of Christmas.
6) By the middle ages we see a richly developed liturgical season filled with special readings, prayers, hymns and devotions.