The Church’s liturgical year began with the first week of Advent and a new cycle for the liturgical calendar. We’re in Year A (see the Mass page on 9).
There’s a whole lot of history behind how the liturgical year was decided. Did you know it didn’t always begin at Advent? Chair of the Liturgical Commission Msgr Michael de Verteuil broke it down in simple terms — he had to; hundreds of years are involved! And I wouldn’t like my Editor to give me the look for exceeding my word count…again.
Why does the Church’s liturgical year begin in Advent? Msgr de Verteuil said in around the fourth century there was a period of preparation in Spain and Gaul (present-day France, Belgium, Luxembourg and parts of the Netherlands), territories named by the Romans where Celtic Gauls lived (source: www.ancient.eu).