The word Lent itself is derived from the Anglo-Saxon word meaning "Spring," and another word which also was the word for "March," the month in which the majority of Lent falls. Fr. Saunders writes:
"Since the earliest times of the Church, there is evidence of some kind of Lenten preparation for Easter. For instance, St. Irenaeus (d. 203) wrote to Pope St. Victor I, commenting on the celebration of Easter and the differences between practices in the East and the West: "The dispute is not only about the day, but also about the actual character of the fast. Some think that they ought to fast for one day, some for two, others for still more; some make their 'day' last 40 hours on end. Such variation in the observance did not originate in our own day, but very much earlier, in the time of our forefathers" (Eusebius, History of the Church, V, 24). When Rufinus translated this passage from Greek into Latin, the punctuation made between "40" and "hours" made the meaning to appear to be "40 days, twenty-four hours a day." The importance of the passage, nevertheless, remains that since the time of "our forefathers" -- always an expression for the apostles -- a 40-day period of Lenten preparation existed. However, the actual practices and duration of Lent were still not homogenous throughout the Church.
Lent becomes more regularized after the legalization of Christianity in A.D. 313. The Council of Nicea (325), in its disciplinary canons, noted that two provincial synods should be held each year, "one before the 40 days of Lent." St. Athanasius (d. 373) in this "Festal Letters" implored his congregation to make a 40-day fast prior to the more intense fasting of Holy Week. St. Cyril of Jerusalem (d. 386) in his Catechectical Lectures, which are the paradigm for our current RCIA programs, had 18 pre-baptismal instructions given to the catechumens during Lent. St. Cyril of Alexandria (d. 444) in his series of "Festal Letters" also noted the practices and duration of Lent, emphasizing the 40-day period of fasting. Finally, Pope St. Leo (d. 461) preached that the faithful must "fulfill with their fasts the Apostolic institution of the 40 days," again noting the apostolic origins of Lent. One can safely conclude that by the end of the fourth century, the 40-day period of Easter preparation known as Lent existed, and that prayer and fasting constituted its primary spiritual exercises."
Others have speculated that Lent may have originally only been for believers preparing for baptism.