The Liturgical Day
In the first chapter of Genesis we hear the refrain six times: "And there was evening and there was morning, the first day" ... "There was evening and there was morning, the second day" etc. Based on this refrain the church follows liturgical time, which is different than calendar time. For liturgical purposes the new day begins at sunset, and runs through to the next.
Such accounting of time is a custom, and not a law. But it is instructive. It helps us keep the Creation in mind. We and our world were created by God, and are sustainted by him. We did not evolve from lower froms of life as is commonly believed today.
It reminds us that the church "marches to the beat of a different drum." St. John writes in his first epistle: Do not love the world or the things in the world. If anyone loves the world, the love of the Father is not in him. (1 John 2:15)
It also reminds us that: In the fullness of time God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave, but a son, and if a son, then an heir through God. Galatians 4:4-7
The same can be said of the liturgical year which, for the church, begins on the 1st Sunday of Advent falling between November 27th and December 3rd. (This year Advent started as late as it can, which is why this morning we celebrated the 4th Sunday of Advent, but beginning at 5:02 PM (local time) we celebrate Christmas.
Such things keep us focused on the things of our great God and Savior Jesus Christ.