Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XXV. The answer to the effect that the fast of Lent has reference to the tithe of the year.

Theonas: Although the pious simplicity of some folks would put aside a question on this subject, yet because you are more scrupulous in your examination of those things which another would consider unworthy to be asked about, and want to know the whole truth of this observance of ours and the secret of it, you shall have a very clear reason for this also, that you may still more plainly be convinced that our predecessors taught nothing unreasonable. By the law of Moses the command propounded to all the people generally was this: “Thou shalt offer to the Lord thy God thy tithes and firstfruits.” 2207 And so, while we are commanded to offer tithes of our substance and all our fruits, it is much more needful for us to offer tithes of our life and ordinary employments and actions, which certainly is clearly arranged for in the calculation of Lent. For the tithe of the number of all the days included in the revolving circle of the year is thirty-six days and a half. But in seven weeks, if Sundays and Saturdays are subtracted, there remain thirty-five days assigned for fasting. But by the addition of Easter Eve when the Saturday’s fast is prolonged to the cock-crowing at the dawn of Easter Day, not only is the number of thirty-six days made up, but in regard to the tithe of the five days which seemed to be over, if the bit of the night which was added be taken into account nothing will be wanting to the whole sum.

Footnotes

Exod. xxii. 29.