Celtic Advent in the pre Nicene church, early Christianity that pre-dates the Nicene Creed, the roadmap of what Christians, Catholic and Protestants believe.  was very different than what we celebrate in the modern time. First Celtic Christianity participated and flourished in this period right up though the middle ages and had its own unique way of celebrating the season of Celtic Advent.

The Celtic Church was given this name to describe the period before the missionaries arrived. St. Patrick a famous saint arose out of the Celtic Christianity era for Ireland Wales and Scotland. He was a British missionary, maybe the most famous of all the Saints to rise out of this time approximately the 5^th century. Although, from a historical perspective ironic, Ireland was evangelized and converted by the British Missionaries of which St. Patrick was a participant.

Early periods of advent have roots back to the Gauls and their Monks who seems to fashion the period to mimic advent. They did not have a Celtic advent wreath but started the period of fasting on Nov 15^th. This season kicked off with a Mass. This was defined as a period of fasting for 40 days and atonement. It was a reflective period that many who were preparing for baptism would use this for purification.  The period of time is still observed throughout the eastern orthodox faiths. And in the Roman Catholic Church this period of purification and enlightenment was moved to lent and still observed within the present time as part of the RCIA program that does use the tenants of the early church to bring people into the church. This was restructured this way after Vatican II. 

In the 4^th century, Advent, mimicking Lent seems to start to change a bit. Although it still was penitential in nature and was still a fasting period, it also started to become a period of preparation for the Second Coming of Christ. This tradition still is with us to this day. As we move though the present season of Advent, we use our advent wreath, we mark our salvation history. We then look to preparing for the Christ Child and then on to Christs second coming. We still have a dedication to prayer and penance in the present church.

Although its is difficult to pinpoint the true start of advent during this period of church history. It is generally believed there are it is a blend between the Roman customs and also the Gauls, or modern day France. There was a group of Celtic monks who help blend the period of fasting and borrowed the 40 days from Lent and added the feast of Martin of Tours on November 11^th.  Martin was the founder of the Gaul Monasteries