Triodion

The Triodion, also called the Lenten Triodion, is a liturgical book used by the Eastern Orthodox and Byzantine Catholic churches. The book contains the propers for the fasting period preceding Pascha and for the weeks leading up to the fast.
The canons for weekday Matins in the Triodion contain only three odes and so are known as "triodes", after which the Triodion takes its name. The period that the book covers extends from the Sunday of the Publican and Pharisee, and concludes with the Midnight Office of Holy Saturday.
The Triodion contains the propers for:

The Pre-Lenten period, begins with a week in which there is no fasting, including on Wednesdays and Fridays, which are normally kept as fast days throughout the year.
The Apokreo marks the change of diet to the fasting practice of Lent: meat is no longer eaten after the "First Apokreo Sunday". coincides with the Carnival celebrations which, although officially discouraged by the Church as pagan remnants, are very popular.
The week following Apokreo is Tyrinĕ which culminates on Tyrinē Sunday and just before Clean Monday, where milk and dairy products can be consumed but not meat or eggs.
"Clean Monday" signifies the first day of "The Forty Days of Great Lent". This period also coincides with the springtime birth of new lambs.
Great and Holy Week

In the edition of the Lenten Triodion used by the Old Believers and those who follow the Ruthenian recension, the contents of the Triodion end with the service of Lazarus Saturday and do not contain the services of Holy Week, which are to be found in the Pentecostarion.