The times within which the sacred offices may be performed. In
the Roman Catholic Church they are seven- viz. matins, prime,
tierce, sext, nones, vespers, and compline. Prime, tierce, sext,
and nones are the first, third, sixth, and ninth hours of the
day, counting from six in the morning. Compline is a corruption
of completorium (that which completes the services of the day).
The reason why there are seven canonical hours is that David
says, “Seven times a day do I praise thee” (Psalm oxix. 164). In
England the phrase means the time of the day within which
persons can be legally married, i.e. from eight in the morning
to three p m. Source: Dictionary of Phrase and Fable, E. Cobham
Brewer, 1894 More on Canonical Hours from Infoplease: