Q. 1. What is a "Fast Day?" There is nothing specific about it in the Catechism of the Catholic Church. The Canon Law speaks of it very generally.
A. 1. The Code Canon Laws and the Catechism of the Catholic Church were written for the Universal Church (meaning the entire world). Because of that, some of the particulars regarding "fasting" are left up to the individual Conferences of Catholic Bishop to provide the instructions for the faithful of their country.
On the matter of "penance," the Code of Canon Law states:
"Days of Penance
Can. 1249 The divine law binds all the Christian faithful to do penance each in his or her own way. In order for all to be united among themselves by some common observance of penance, however, penitential days are prescribed on which the Christian faithful devote themselves in a special way to prayer, perform works of piety and charity, and deny themselves by fulfilling their own obligations more faithfully and especially by observing fast and abstinence, according to the norm of the following canons.
Can. 1250 The penitential days and times in the universal Church are every Friday of the whole year and the season of Lent.
Can. 1251 Abstinence from eating meat or some other food according to the prescripts of the conference of bishops is to be observed on, of abstinence binds those who have completed their fourteenth year of age. The law of fasting, however, binds all those who have attained their majority until the beginning of their sixtieth year. Nevertheless, pastors of souls and parents are to take care that minors not bound by the law of fast and abstinence are also educated in a genuine sense of penance.
Can. 1253 The conference of bishops can determine more precisely the observance of fast and abstinence as well as substitute other forms of penance, especially works of charity and exercises of piety, in whole or in part, for abstinence and fast."
Prior to Vatican II, the Fasting Guidelines from the New Marian Missal, published by Angelus Press, stated the following for the US:
"Abstinence: All Catholics seven years and older are obliged to observe the Law of Abstinence. On days of complete abstinence flesh meat, soup or gravy made from meat are not permitted at all. On days of partial abstinence flesh meat, soup or gravy made from meat ar permitted once a day at the principal meal.
Complete abstinence is to be observed on all Fridays of the year, Ash Wednesday, Vigils of the Immaculate Conception and Christmas, and on Holy Saturday. Partial abstinence is to be observed on Ember Wednesdays and Saturdays, and on the Vigil of Pentecost.
Fasting: All Catholics from the completion of their twenty-first year to the beginning of their sixtieth year are bound to observe the Law of fast. The days of fast are the weekdays of Lent, Ember Days, the Vigils of Pentecost, the Immaculate Conception, Christmas. Only one full meal is allowed on a day of Fast. Two other meatless meals are permitted. These meals should be sufficient to maintain strength in accordance with each one's needs. Both of these meals, or collations, together, should not equal one full meal.
It is permissible to eat meat at the principle meal on a Fast Day except on Fridays, Ash Wednesday, and the Vigils of Immaculate Conception, Christmas, and Holy Saturday.
Solid foods between meals is not permitted. Liquids, including coffee, tea, milk and fruit juices are allowed.
In connection with problems arising from the Laws of Fast and Abstinence, a confessor or priest should be consulted. Dispensations may be granted for a serious reason concerning health or the ability to work."
In Canada, the fasting guidelines read:
"Fasting Days: All days in Lent, except Sundays, the Ember Days, the Vigils of Whitsunday, of All Saints, of the Immaculate Conception and of Christmas Day.
Abstinence Days: The Ember Days, the Vigils mentioned above, the Wednesdays of Lent, Holy Saturdays forenoon, and all Fridays, except Days of Obligation."
Further information can be obtained from the EWTN website.