The Four Books Used
Four books contain the festal services necessary for the liturgical year.
The Menaion contains the feasts of the Lord, the Mother of God, and the Saints, for every day of the year.
The Triodion contains the services for Great Lent, the three weeks before, and Holy Week.
The Pentecostarion contains the feasts of Pascha through Pentecost and All Saints.
The Octoëchos provides eight one-week cycles of services in one of each of the eight tones.
Of these, the Menaion’s feasts are immovable, while the others are moveable.
Immovable feasts occur on the same calendar day every year; the moveable feasts can fall within a certain range of dates.
The discussion of the moveable feasts is somewhat complex, so we will begin with the simpler immovable feasts of the Menaion in the next diagram.