The Celtic Advent period lasts for forty days. Nov. 15-Dec. 24. The Dates are the same for Eastern Orthodox Advent (Nativity Fast, Winter Lent, or the Christmas Lent).
During the time of ancient Celtic Christianity, the entire Church, both Western (including the Celtic Christians), and Eastern (the Orthodox Communions, Oriental Churches, and Eastern Rite Roman Catholics) all celebrated a longer Advent Season as a lesser Lenten fast. Advent seems to have been a result of the observance of the Celtic monks in Gaul,
It began on the same date every year on November 15th (Orthodox Churches still observe it as beginning on this day Observance of Advent appears to have taken place since the 4th Century (300's A.D.) Like Lent, it originally was a season when new Christians studied in preparation for being baptized. In the early Middle Ages Advent was the Season of preparing oneself for the Second Coming of Christ. It was a season of repentance and dedication to prayer.
a short Northumbrian Advent compline that can be used nightly until the 17th of December and the begining of the Advent Antiphons
Celtic Advent liturgy that can be used Sundays leading up to christmas
Living Water Acient Well explanation of Advent