Beginning with Pascha, the Resurrection of Christ, the Church’s liturgical calendar is built around a cycle which repeats every eight weeks. Each week, the hymns are written in a different Tone or family of melodies. For each of the eight weeks, certain hymns are appointed for each service, regardless of what the date is. For this reason, the daily services are always different from year to year, as the hymns for the feast or saint of the day combine with the hymns of the tone.
The Octoechos (the Book of Eight Tones) collects hymns for every Sunday and weekday. It contains the variable hymns for Vespers, Compline, the Midnight Office, the Hours, Matins, and the Divine Liturgy.