The American Church Dictionary and Cycopedia
An office of deep devotion and contrition to be used on Ash Wednesday, which was added to the Prayer-book at its last revision in 1892. Its place in the service is during the latter part of the Litany. It may be used on other days at the discretion of the minister. (See ASH WEDNESDAY.)
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Miller, William James. Entry for 'Penitential Office'. The American Church Dictionary and Cyclopedia. https://www.studylight.org/dictionaries/eng/acd/p/penitential-office.html. 1901.