Leo_1St. (Pope) Leo the Great from Sermon 16, Section II (On the Fast of the Tenth Month) [this period is similar to our Advent]

And while all seasons are opportune for this duty, beloved, yet this
present season is specially suitable and appropriate, at which our holy
fathers, being Divinely inspired, sanctioned the Fast of the tenth
month, that when all the ingathering of the crops was complete, we
might dedicate to God our reasonable service of abstinence, and each
might remember so to use his abundance as to be more abstinent in
himself and more open-handed towards the poor. For forgiveness of sins
is most efficaciously prayed for with almsgiving and fasting, and
supplications that are winged by such aids mount swiftly to God’s ears:
since as it is written, "the merciful man doeth good to his own soul
," and nothing is so much a man’s own as that which he spends on his
neighbour. For that part of his material possessions with which he
ministers to the needy, is transformed into eternal riches, and such
wealth is begotten of this bountifulness as can never be diminished or
in any way destroyed, for "blessed are the merciful, for God shall have
mercy on them ," and He Himself shall be their chief Reward, who is the Model of His own command.

Read the whole thing at NewAdvent

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