The history of the Lenten fast that Catholics observer dates back into the ancient times but was far stricter than it was today. However, there was no set pattern of rule the way there is today. Without a central church the rituals of Lent varied greatly. Some prohibited all animal products, while others did allow fish and others allowed fowl.  There were some places that also added fruits and eggs and only allowing bread. And then there were the people that just abstained from food for a full day, some also only ate one meal a day while sill there were others that would abstain though 3 o.clock in the afternoon.  By far the most popular ritual however was the ritual to note eat all day and then have a small meal that did not include meat or alcohol.  

After the formation of the central church, in the Middle Ages, meat, egg and dairy were singled out to the foods to abstain from.  The thought of this these were bought more pleasure than fish according to Thomas Aquinas.  In the Summa Theologica, St. Aquinas lays out the rules of the Lenten fast.  He says that the fast was instated to curb the strong desire or concupiscence of the flesh.  Additionally it is said that the drinking of wine increases concupiscence and thus why that also was added.  Fish was thought to be far less desirable and thus making it much more apt to .bridle concupiscence. said Aquinas.

However, because of the corruption of the Catholic Church in the Middle Ages, often a donation allowed a special dispensation from dairy products. A dispensation is by definition is the act of an authority, in this case the church, to make an exception to laws.  This was very common in the Middle ages and one of the reasons that lead to the Protestant Reformation, lead by Martin Luther. Additionally, during the Crusade, there was an allowance to allow dairy products and eggs during Lent. What the Christian solider had to do was agree to   help and fight in the war against Islam.

In the modern day maybe of the old world rituals are still observed. In the West, the rituals are far more relaxed. However in the Orthodox and Eastern Catholic Churches are far stricter and adhere to more old world traditions, meals consisting of mostly vegetarian meals.  The Lenten practices in the West, centered on the Roman Catholic Church has been refined to be no meat on Fridays, however dairy products are now allowed. What the Catholic Church has defined is as meat is Ungulates, or meat of a hoofed animal.  Expanding this is also a more customary fast on Ash Wednesday. The Catholic Church defines more customary as been a full day of fast which includes no meat. Each person is allowed one full meal on this day, mindful of no meat, and they may also have two smaller meals if its needed.  The other day in the Catholic Church which also is a day of fast is Good Friday, one of the most holy days in the church history.