Almighty God, give us grace that we may cast away
the works of darkness, and put upon us the armour of
light, now in the time of this mortal life, in which thy
Son Jesus Christ came to visit us in great humility;
that in the last day, when he shall come again in his
glorious majesty to judge both the quick and the dead,
we may rise to the life immortal, through him who
liveth and reigneth with thee and the Holy Ghost, now
and ever. Amen.
Today is the first day of the liturgical year in the Western Catholic calendar and of the first season -- Advent. The English word Advent is derived from the Latin, advenio, which means "to come to."
According to present usage, Advent is a period beginning with the Sunday nearest to the feast of St. Andrew the Apostle (30 November) and embracing four Sundays. The first Sunday may be as early as November 27, and then Advent has twenty-eight days, or as late as December 3, giving the season only twenty-one days. As you will see, today is the latest day upon which Advent can begin.
As the Catholic Encylopedia points out, the faithful are admonished during this time
+ to prepare themselves worthily to celebrate the anniversary of the Lord's coming into the world as the incarnate God of love,
+ thus to make their souls fitting abodes for the Redeemer coming in Holy Communion and through grace, and
+ thereby to make themselves ready for His final coming as judge, at death and at the end of the world.
As the season of Lent is a time for reflection and preparation for Easter, the Feast of the Resurrection, Advent prepares us for Christmas, the Feast of the Nativity, when God takes upon Himself our human nature and humbles himself to be born as a child in a simple family.
The invocation in this collect, for us to "cast away the works of darkness, and put upon us the armour of light" is taken directly from today's Epistle, Romans 13.8ff, and tells us what we are called upon to do.
The works of darkness are the works of the enemy, of Satan, and are reflected in our fallen nature and in our sinfulness. The armour of light is a reference to Jesus. As this is Advent, we are called upon to reflect on the coming of Jesus into the world and His offer to come into us. But only we can choose to receive Him, and in choosing, we "put Him on" as our armour against the deceits and attacks of the Devil.
But before we proceed, we must go to the very beginning of this prayer, and to the key word grace. We may choose to accept Christ as our Protector and Savior, but we cannot do this on our own. We do it only by grace, by accepting his gift of Himself, which is something that we can never earn.
This leads us to the warning of this prayer. If we are to cast away the works of darkness and put on the armour of light, it is something that we must do now, "in this mortal life", in order to prepare ourselves for the Last Day, when He will come to judge us, and when that Judgement will determine whether we are to share with Him in His immortal life.