This most compunctionate Canon-which is truly great not only because of the quantity of Troparia as compared with other canons, but also because of the multitude of mystical thoughts and insights-is the excellent composition of Saint Andrew of Crete (July 4). Examining the entire history of the Holy Scriptures, both Old and New, and artistically fitting it together into one sacred song, this divine Father incites every soul to imitate the good and to flee from evil, and to return to God by means of repentance.
The Great Canon of St Andrew, Bishop of Crete, is the longest canon in all of Orthodox services, and is associated with Great Lent, since the only times it is appointed to be read in church are the first four nights of Great Lent (Clean Monday through Clean Thursday, at Great Compline, when it is serialized) and at Matins for Thursday of the fifth week of Great Lent, when it is read in its entirety (in this latter service, the entire life of St Mary of Egypt is also read).
There is no other sacred hymn which compares with this monumental work, which St Andrew wrote for his personal meditations. Nothing else has its extensive typology and mystical explanations of the scripture, from both the Old and New Testaments. One can almost consider this hymn to be a “survey of the Old and New Testament”. Its other distinguishing features are a spirit of mournful humility, hope in God, and complex and beautiful Trinitarian Doxologies and hymns to the Theotokos in each Ode.
The canon is a dialog between St. Andrew and his soul. The ongoing theme is an urgent exhortation to change one’s life. St Andrew always mentions his own sinfulness placed in juxtaposition to God’s mercy, and uses literally hundreds of references to good and bad examples from the Old Testament and New Testament to “convince himself” to repent.
A canon is an ancient liturgical hymn, with a very strict format. It consists of a variable number of parts, each called an “ode”. Most common canons have eight Odes, numbered from one to nine, with Ode 2 being omitted. The most penitential canons have all nine odes. Some canons have only three Odes, such as many of the canons in the “Triodion” (which means “Three Odes”).
In any case, all Odes have the same basic format. An “Irmos” begins each Ode. This is generally sung, and each Irmos has a reference to one of the nine biblical canticles, which are selections from the Old and New Testament, which can be found in an appendix in any complete liturgical Psalter (book of Psalms, arranged for reading in the services). A variable number of “troparia” follow, which are short hymns about the subject of the canon. These are usually chanted, and not sung. After each troparion a “refrain” is chanted. At the end of each Ode, another hymn, called the “Katavasia”, either the Irmos previously sung, or one like it is sung.
The Who’s Who of the Great Canon, here
The text of the service, here