The Machzor is a specialized form of the Siddur, the Prayer Book used by Jews around the world.
The Machzor (or Mahzor) is used by religious Jews on the High Holidays (Yamim Noraim, "The Days of Awe") of Rosh Hashanah, Yom Kippur, and the three "Pilgrim Festivals" (Shalosh Regalim) of Passover, Shavuot, and Sukkot. The Machzor does not include the daily prayers and blessings that one should recite on week days and Sabbath, which are found in the Siddur. The Machzor is among the most widely-circulated and best-known of Jewish books.
The word Machzor comes from Hebrew, and it literally means "cycle".
Developing the Machzor from the Siddur
Some of the earliest formal printed Jewish prayerbooks date from the 10th century; they contain a set order of daily prayers. However, there are many liturgical variations and additions that Jews add to the prayer service for the High Holy Days (Yamim Noraim, "The Days of Awe"), Rosh HaShanah and Yom Kippur. As such, a special Siddur has developed for just this period, the Machzor.
A Machzor contains not only the basic liturgy, but also many piyutim, Hebrew liturgical poems. Many of the prayers in the Machzor, including those said daily or weekly on the Sabbath, have special meleodies sung only on the holidays. The Machzor contains only text and no musical notation and the melodies, some of which are ancient, have been passed down orally.