The Horologion (Chasoslov in church Slavonic), also known as the Book of Hours, is a bound collection containing the services of the Daily Cycle in the Eastern Orthodox Church.

The Daily Cycle of divine services is the recurring pattern of prayer and worship that punctuates each liturgical day.

Monasteries generally serve the entire cycle of services. Some cathedrals do, as well. Most parishes do not.

The Divine Liturgy is not itself a part of the Daily Cycle but is inserted into the cycle, usually after Orthros or the Sixth Hour. In some cases it is combined with Vespers. The service of Typika is related to the Divine Liturgy and is often celebrated on days the Divine Liturgy is not appointed.

The Daily Cycle follows this pattern:

Vespers (sunset) Compline (after-dinner) - also known as Apodeipnon Midnight Office (12:00am) - also known as Mesonyktikon Orthros (sunrise) - also known as Matins First Hour (6:00am) Third Hour (9:00am) Sixth Hour (12:00pm) Ninth Hour (3:00pm)

Although each service of the Daily Cycle has a particular time traditionally associated with it, in current practice the services are more commonly served together in one of three aggregations.

The Evening Aggregate usually consists of Ninth Hour, Vespers, and Compline. It commences shortly before sunset. The Dawn Aggregate usually consists of Midnight Office, Orthros, and First Hour. It usually begins shortly before sunrise. The Midday Aggregate consists of Third Hour and Sixth Hour. The Divine Liturgy or Typika may be included in the Midday Aggregate. On days when Great Compline is appointed (e.g., weekdays in Great Lent), it constitutes the entirety of the Evening Aggregate. Vespers is shifted to the Midday Aggregate.

On the eves of Certain Feast Days, a special aggregate, known as the All-Night Vigil, may be served. In such case, the other aggregations may be altered slightly (e.g., including Little Vespers in the Evening Aggregate). Although there is some variance in practice, the All-Night Vigil generally includes at least Great Vespers, Orthros, and First Hour.