Since 1921, there have been two calendars in use within the Orthodox Church: the Julian Calendar and the Revised Julian Calendar (which aligns its fixed holy days with the Gregorian Calendar). At the current time there is a thirteen day difference between the two calendars. This means that those churches which use the New Calendar (Gregorian) will celebrate the feasts on the fixed cycle thirteen days before those who follow the Old Calendar (Julian). The other major annual cycle, the movable cycle is the same for both Old and New Calendar Churches, so all will celebrate Pascha (Easter) on the same day.
The liturgical texts for celebrations on the Menaion are contained in twelve volumes called menaia. Each menaion will contain the services for an entire month. The liturgical year for Eastern Orthodox Christians begins in September, so the Menaion for September is the first volume of the set.
The menaion contains the largest collection of liturgical texts that are used in the Eastern Church and is a very important component of the liturgical books owned by a parish or monastery. Outside of Great Lent, texts in the menaion are used in every one of the Divine Services—with the exception of the Midnight Office—and in the Divine Liturgy (Troparion, Kontakion, Stichera at the Beatitudes, etc.).
There is another volume called the General Menaion which contains services for each type of celebration (Apostles, Martyrs, etc.) with blank spaces for the name of the saint being celebrated. When a parish is not able to afford a complete set of menaia (as often happens in mission situations), or if they do not have the texts for a particular saint they wish to commemorate, it is normal to use the General Menaion to fill in for those services which are missing.
There is also what is called the Festal Menaion, an anthology which contains the texts for those Great Feasts of the Lord or the Theotokos which fall on the fixed cycle (Great Feasts of the moveable cycle are contained in the Lenten Triodion and the Pentecostarion). Some Festal Menaia may also contain feasts of the major saints.