"They will fast"*

Matt.9
[14] Then the disciples of John came to him, saying, "Why do we and the Pharisees fast, but your disciples do not fast?"
[15] And Jesus said to them, "Can the wedding guests mourn as long as the bridegroom is with them? The days will come, when the bridegroom is taken away from them, and then they will fast.

CHAPTER II : DAYS OF PENANCE

Can. 1249 All Christ's faithful are obliged by divine law, each in his or her own way, to do penance. However, so that all may be joined together in a certain common practice of penance, days of penance are prescribed. On these days the faithful are in a special manner to devote themselves to prayer, to engage in works of piety and charity, and to deny themselves, by fulfilling their obligations more faithfully and especially by observing the fast and abstinence which the following canons prescribe.

Can. 1250 The days and times of penance for the universal Church are each Friday of the whole year and the season of Lent.

-Code of Canon Law

*Nota bene:
The Octave of Easter is one long Easter "Day" and therefore the Friday in the Octave of Easter is considered not a Friday but, rather, a Sunday and therefore penance is not observed.
- Haurietis Aquas