Christians familiar only with the Christian West may be surprised to learn that Ash Wednesday and its customs exist only in the Western church. The Eastern churches have other ways of counting the days of Lent, and of beginning this Great Fast.
The Roman Catholic Church counts Holy Week as part of the Lenten Fast, but not the Sundays during the Lenten season. Therefore, in about the 8th Century, it was necessary to add four days to the beginning of Lent to bring the number of days up to the traditional 40. This was the origin of Ash Wednesday.
The Eastern Christian Churches (both Catholic and Orthodox) consider Great and Holy Week as a separate unit with its own Fasting and Abstinence requirements, not technically included in the Great Lent. Lent liturgically concludes on the evening of the 6th Friday of Great Lent, the vigil of Lazarus Saturday. Although we do not fast (restrict the amount of food eaten) on Saturdays and Sundays, we do continue to abstain from certain kinds of foods on the Weekends of Lent. The Saturdays & Sundays of the Great Fast are counted in the total of days, thus bringing the number up to 40, counted from Clean Monday, the first day of Great Lent.