One should read Canons 1249 - 1253 for the actual information on Days of Penance, see:
vatican.va/archive/ENG1104/__P4O.HTM
Code of Canon Law:
vatican.va/archive/cdc/index.htm
Rules for Fasting and Abstinence
- All persons who have completed their 14th year are bound by the law of abstinence; all adults are bound by the law of fast from the completion of their 18th year of age up to the beginning of their 60th year (inclusive).
- Complete Abstinence are days which forbid the eating of meat as well as soup and gravy made from meat.
- Complete Fast are days where only one full meal is allowed with two other meatless meals. The two other meatless meals should be sufficient for to maintain strength but both together should not be equal to another full meal.
- Days of Fast and Abstinence
Complete Fast and Abstinence is to be observed under pain of mortal sin only on Ash Wednesday and Good Friday.
Abstinence is to be observed on all Fridays of Lent and on all Fridays during the year (but without fault or sin if one fails to do so).
All Fridays throughout the year and the time of Lent are penitential days and times throughout the Universal Church (Can. 1250).
First place is given to abstinence from flesh meat on all Fridays. However, if abstinence is not observed, then some other works of self-denial or personal penance is to be observed. Examples would be doing voluntary work in hospitals, visiting the sick, serving the needs of the aged and the lonely, instructing youth in the Faith, etc. Examples of Lenten devotions would be sermons, Stations of the Cross, the Rosary, etc.
On other Fridays (non-Lenten) during the year, one is allowed to commute abstinence into another form of penance (e.g., Way of the Cross).
On the weekdays of Lent, daily Mass is strongly recommeded as well as a self-imposed observance of fasting
Whenever a solemnity (First Class Feast) falls on Friday, abstinence is dispensed (Can. 1251).