The 40 days of Great Lent begins on the Monday after Cheesefare Sunday, Pure Monday (Kathara Deftera), which this year (2018) falls on February 19. On Friday the 23rd of February we start the Service of Salutations to the Theotokos (Chairetismoi).
It is important to note that the Church leads us to this point gradually during the preparatory period of the first three weeks of the Triodion.
During the first week, She allows us to eat everything, even on Wednesdays and Fridays. During the second week, we can eat everything, except for meat on Wednesday and Friday. During the third week, we are no longer allowed to eat meat, but we can eat fish, eggs and dairy products.
The Church has thereby introduced us gradually into the more severe fast of Great Lent which begins on Kathara Deftera, when the faithful embrace a totally vegetarian diet.
The custom of preparing Christians for the celebration of Pascha through fasting and prayer is very ancient, but neither the length nor the type of fast was strictly specified during the first centuries.
For instance, Saint Irenaeus, the Bishop of Lyons at the end of the 2nd Century AD, tells us that some fasted for one day, others for two days, others for more, and still others for only 40 hours. Socrates, the 5th Century church historian, tells us that some did not eat meat while some ate poultry and fish, and while still others ate only vegetables.
In the 4th Century, however, the fast of the Paschal Cycle began to take a more specific form, at least in regard to length. It was extended to 40 days in commemoration of the 40 days, which the Lord fasted in the wilderness, hence the term Sarakoste (i.e., Lent).