From St. Augustine, on daily Mass:
I promised you, who have now been baptized, a sermon in which I would explain the Sacrament of the Lord's Table, which you now look upon and of which you last night were made participants. You ought to know what you have received, what you are going to receive, and what you ought to receive daily. (Sermon 227)
Thus [Christ] is both the Priest who offers and the Sacrifice offered. And He designed that there should be a daily sign of this in the sacrifice of the Church, which, being His body, learns to offer herself through Him. (City of God X, 20)
There are other things, however, which are different in different places and countries: e.g., some fast on Saturday, others do not; some partake daily of the body and blood of Christ, others receive it on stated days: in some places no day passes without the sacrifice being offered; in others it is only on Saturday and the Lord’s day, or it may be only on the Lord’s day. (Epistle LIV, 2)
Some one may say, “The Eucharist ought not to be taken every day.” You ask, “On what grounds?” He answers, “Because, in order that a man may approach worthily to so great a sacrament, he ought to choose those days upon which he lives in more special purity and self-restraint; for ‘whosoever eateth and drinketh unworthily, eateth and drinketh judgment to himself.’” ... If, however, his sins are not so great as to bring him justly under sentence of excommunication, he ought not to withdraw himself from the daily use of the Lord’s body for the healing of his soul.” (Epistle LIV, 4)
For the wolf will come — not man, but the devil, who has very often perverted to apostasy believers to whom the daily ministry of the Lord’s body was wanting... (Epistle CCXXVIII, 6)
The sacrament of this thing, namely, of the unity of the body and blood of Christ, is prepared on the Lord’s table in some places daily, in some places at certain intervals of days, and from the Lord’s table it is taken, by some to life, by some to destruction: but the thing itself, of which it is the sacrament, is for every man to life, for no man to destruction, whosoever shall have been a partaker thereof. (Tractates on the Gospels of John XXVI, 15)
From Pope St. Gregory the Great, on multiple Masses in a day:
Because [by the Lord's bounty] I am going to celebrate the eucharist three times today, I can comment only briefly on the Gospel lesson. But [our Redeemer's] birthday compels me to say something, however short. (Homily 7, in Forty Gospel Homilies)
(H/T to Fr. Daren Zehnle)