Saint Mark's feast day is April 25th.
The Gospels are probably the most important written works of all of Christianity. It is from their words, that the Word of God has been carried in writing to millions of people throughout the centuries. Each of these writers is represented by a creature.
As one of the four Gospel writers, and one of the seventy disciples of Jesus Christ, Saint Mark is considered by the Church to represent the Lion. This was taken from a vision seen by the Prophet Ezekiel around 625 years B.C.:
And every one had four faces: the first face was the face of the cherub, and the second face was a human face, and the third the face of a lion, and the fourth the face of an eagle. - Ezeckiel 10:14
More than 600 years later, Saint John the Evangelist in his book "Revelations" has a parallel vision:
In the center, around the throne, were four living creatures, and they were covered with eyes, in front and in back. 7 The first living creature was like a lion, the second was like an ox, the third had a face like a man, the fourth was like a flying eagle. Each of the four living creatures had six wings and was covered with eyes all around, even under its wings. Day and night they never stop saying: “‘Holy, holy, holy is the Lord God Almighty,’ who was, and is, and is to come.” - Revelations 4:6-8
Saint Mark is known as a lion for two reasons: 1) He describes Saint John the Baptist as a lion, "crying out in the wilderness"; and 2) because Jesus saved Saint Mark and his father from being attacked by lions after the Resurrection when they were on their way to Jordan. St. Mark's father converted to Christianity.
Saint Mark is the patron of notaries and is also known for having established a Church in Alexandria.