Leo_1

St. (Pope) Leo the Great from Sermon 12, Section IV ( On The Fast of the Tenth Month) [this period is similar to our Advent]

But there are three things which most belong to religious actions,
namely prayer, fasting, and almsgiving, in the exercising of which
while every time is accepted, yet that ought to be more zealously
observed, which we have received as hallowed by tradition from the
apostles: even as this tenth month brings round again to us the
opportunity when according to the ancient practice we may give more
diligent heed to those three things of which I have spoken. For by
prayer we seek to propitiate God, by fasting we extinguish the lusts of
the flesh, by alms we redeem our sins: and at the same time God’s image
is throughout renewed in us, if we are always ready to praise Him,
unfailingly intent on our purification and unceasingly active in
cherishing our neighhour. This threefold round of duty, dearly beloved,
brings all other virtues into action: it attains to God’s image and
likenessand unites us inseparably with the Holy Spirit. Because in
prayer faith remains stedfast, in fastings life remains innocent, in
almsgiving the mind remains kind. On Wednesday and Friday therefore let
us fast: and on Saturday let us keep vigil with the most blessed
Apostle Peter, who will deign to aid our supplications and fast and
alms with his own prayers through our Lord Jesus Christ, who with the
Father and the Holy Ghost lives and reigns for ever and ever. Amen.

Read the whole thing at New Advent.

Image used above and information on St. Leo at Doctors of the Catholic Church: St. Leo the Great.