This differs significantly from other Christian traditions, which celebrate Advent four Sundays before Christmas, and celebrate the Annunciation (the day on which Mary was told by the Angel Gabriel that she would conceive Jesus) on 25 March, a materialist nine months of biological gestation prior to Christmas.
For the Reform, the historical placement of the Annunciation is not as important as the inspirational role it plays as part of the Nativity story. By observing this herald of the Nativity together with Advent, AUR brings the entire narrative of the birth of Jesus together in one ritual season, setting aside December as a month of preparing for new beginnings: the beginning of the life of Christ, the beginning of the era of the Tree of Life, and the beginning of the new year when December finally turns over to January.
Reform Unitarian Advent is also the traditional feast day of St. Eligius, patron of goldsmiths, giving us the start of the first Dozen of the Christmas season: the Twelve Days of Gold celebrating Mary as the Mother of Jesus, culminating with the feast day of Our Lady of Guadalupe on December 12th.
Now is the time for unlit Christmas decorations, and for placing Mary and the Angel in the crèche!