The roots of these practices can be found in the Old Testament. At the time of Jesus, observant Jews would have observed fasting days on Mondays and Thursdays. The early Christians changed these fasting days to Wednesday and Friday. The habit of fasting before Easter developed gradually. In 331, St. Athanasius encouraged his flock to fast for 40 days prior to the stricter fast of Holy Week. By the time of Gregory the Great in the 6th century, Rome observed a fast of six weeks made up of six days (Sundays excluded) for a total of 36 fast days. This was referred to as a “spiritual tithing” as 36 is approximately one-tenth of the year.
By the Middle Ages, the traditional fast had become known as the “Black Fast.” This was a fast in which strictly no more than one meal, without meat, dairy, oil or wine, could be eaten. This meal was usually taken in the evening. In the 10th century, this was relaxed to allow the meal to be eaten at 3 PM. By the 14th century, the meal was allowed at midday, and this was soon followed by the addition of a collation, or snack, in the evening. In the 19th century, a morning collation was added.
In the early 20th century, fasting was observed throughout Lent with abstinence observed only on Fridays and Saturdays. However, there were exceptions to this based on country or culture. For example, Ireland observed abstinence on Wednesdays instead of Saturdays and some colonies had different rules for native-born Catholics versus European-born Catholics. The Church also observed other times of fasting and abstinence. These included Advent, Ember Days, Rogation Days, Fridays throughout the year and vigils of important feasts such as Pentecost, All Saints’ Day, Christmas, the Immaculate Conception and the Assumption. These strict rules were gradually relaxed throughout the 20th century until they were replaced by the current rules, which are rooted in Pope Paul VI’s 1966 Apostolic Constitution.