The Church enjoins as fasts all Fridays in the year, except Christmas-day; which, though it fall upon a Friday, is celebrated as a day of rejoicing.

The primitive Church observed Friday as a Fast, in commemoration of our Saviour’s sufferings, who was crucified on this day of the week. On no day can we with more propriety humble ourselves on account of our sins, than on that when the blessed JESUS humbled himself even to the death upon the cross for us, miserable sinners.

[Excerpt from John Henry Hobart, A Companion for the Book of Common Prayer, Containing an Explanation of the Service (1859), 113.]