Paenitemini

FASTING AND SELF-MORTIFICATION are nearly universally observed as necessary means to obtaining a higher end — and an obvious example now are the Olympic athletes, who would not even get to the Games were it not for years of self-denial.  The world loves fasting and self-moritifcation for the ends of money or pleasure, but hates it when Catholics do it for a spiritual end. That this hate is directed solely to the Church shows that this hate has its origins in Hell. So we must keep up the effort at denying the world, the flesh, and the devil through penance.

Current norms on fasting derive from the Apostolic Constitution Paenitemini of Pope Paul VI. From the text:

...following the Master, every Christian must renounce himself, take up his own cross and participate in the sufferings of Christ. Thus transformed into the image of Christ's death, he is made capable of meditating on the glory of the resurrection. Furthermore, following the Master, he can no longer live for himself, but must live for Him who loves him and gave Himself for him. He will also have to live for his brethren, completing "in his flesh that which is lacking in the sufferings of Christ...for in the benefit of his body, which is the church."

Penance supports both the individual and the ecclesial community, helping to restore both to Christ.

The Holy Father teaches:

True penitence, however, cannot ever prescind from physical asceticism as well. Our whole being in fact, body and soul, (indeed the whole of nature, even animals without reason, as Holy Scripture often points out) must participate actively in this religious act whereby the creature recognizes divine holiness and majesty. The necessity of the mortification of the flesh also stands clearly revealed if we consider the fragility of our nature, in which, since Adam's sin, flesh and spirit have contrasting desires. This exercise of bodily mortification-far removed from any form of stoicism does not imply a condemnation of the flesh which sons of God deign to assume. On the contrary, mortification aims at the "liberation" of man, who often finds himself, because of concupiscence, almost chained by his own senses. Through "corporal fasting" man regains strength and the "wound inflicted on the dignity of our nature by intemperance is cured by the medicine of a salutary abstinence."

We must not fall into the philosophical traps of materialism — where we think that only the material body itself is important — or of excessive spiritualism, where we view the body as as prison-house of the soul, which needs to be transcended.

Penance must never be done for show, or merely done because “that's what Catholics do”, which is particularly a danger with wearing Ash Wednesday ashes.

Against the real and ever recurring danger of formalism and pharisaism the Divine Master in the New Covenant openly condemned—and so have the Apostles, Fathers and supreme pontiffs—any form of penitence which is purely external. The intimate relationship which exists in penitence between the external act, inner conversion, prayer and works of charity is affirmed and widely developed in the liturgical texts and authors of every era.

We shouldn't do penance for mere external reasons, but we should not refrain from penance because it is external. Externals are important, but only if they are means to a higher end. Regarding the ashes of Ash Wednesday, it has been said that if you are proud to show off your ashes, then you should remove them, and if you are embarrassed by showing the ashes, then you should keep them on.

Pope Paul VI reminds us of the three penitential practices of prayer, self-mortification, and alms giving, and suggests that episcopal conferences adjust the norms of the practice, depending on local economic conditions: for example, the rich could benefit more from self-denial. The norms given by this apostolic constitution are generous in their mildness, but we ought to be generous in our penance.

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