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