The Baltimore Catechism reminds us:
Q. 1342. When do fast days chiefly occur in the year?In the Early Church, as in the practice still amongst some people in Catholicism and Eastern Orthodoxy, fasting is done on Wednesdays and Fridays. It is done on Wednesdays in memory of our Lord’s betrayal by Judas and on Friday, since it was on Friday that Christ died.
A. Fast days chiefly occur in the year during Lent and Advent, on the Ember days and on the vigils or eves of some great feasts. A vigil falling on a Sunday is not observed.
Q. 1343. What do you mean by Lent, Advent, Ember days and the vigils of great feasts?
A. Lent is the seven weeks of penance preceding Easter. Advent is the four weeks of preparation preceding Christmas. Ember days are three days set apart in each of the four seasons of the year as special days of prayer and thanksgiving. Vigils are the days immediately preceding great feasts and spent in spiritual preparation for them.
Fasting in Lent is an ancient practice with mention of it going back to at least the 2nd century. St. Athanasius in 331 wrote strongly in support of this fast of 40 days, which at that time was before the required and stricter fast of Holy Week.
As of the reign of Pope St. Gregory the Great (590 – 604) there were six weeks of six days of fasting observed for Lent in Rome. The result was 36 days of fasting. As 40 is a Biblical number for fasting as observed in the Old Testament, the practice began of beginning Lent on the preceding Wednesday, that we know of as Ash Wednesday, in order for 40 days of Lenten fasting to be observed.
During this ancient time, the practice of fasting allowed only one meal a day to be eaten (as is the current practice); however, the meal was in these ancient times only to be eaten in the evening.
These days were at one time observed with a strict fast no more than one meal, without meat, dairy, oil, or wine. In the 10th century the custom of taking the only meal of the day at three o'clock was introduced. In the 14th century the meal was allowed at mid-day, and soon the practice of an evening collation (snack) became common. A morning collation was introduced in the early 19th century.
In the early 1900s, the Law of the Church required fasting on all days of Lent but abstinence from meat was required only on Fridays and Saturdays. However, a common practice called partial abstinence was observed, which permitted meat only once a day at the principal meal. Unique exceptions to what constituted meat differed in certain countries (e.g. capybara meat is permitted in South American countries while other meat is forbidden). In such a way, the uniqueness of an individual culture is retained and still yet forms part of the One Body of Christ.
In the early 1900s, as observed in the reading previously, there were additional days of fasting and/or abstinence in the year including the Ember Days, days of Advent, Rogation Days, Vigils of important feastdays, and the like.
Fridays and Saturdays in Advent were days of abstinence, and until early in the 20th century, the Fridays of Advent were also days of fasting.
Ember Days, as explained by the New Advent Catholic Encyclopedia:
Ember days (corruption from Lat. Quatuor Tempora, four times) are the days at the beginning of the seasons ordered by the Church as days of fast and abstinence. They were definitely arranged and prescribed for the entire Church by Pope Gregory VII (1073-1085) for the Wednesday, Friday, and Saturday after 13 December (S. Lucia), after Ash Wednesday, after Whitsunday, and after 14 September (Exaltation of the Cross). The purpose of their introduction, besides the general one intended by all prayer and fasting, was to thank God for the gifts of nature, to teach men to make use of them in moderation, and to assist the needy. The immediate occasion was the practice of the heathens of Rome. The Romans were originally given to agriculture, and their native gods belonged to the same class.
If you are not already fasting for the 40 days of Lent, please do try to join in the traditional Ember Day Lenten Fast.