The Conferences Of John Cassian by John Cassian
Chapter XXIV. A question on the different ways of keeping Lent.
Germanus: What is the reason why Lent is kept for six weeks, while in some countries a possibly more earnest care for religion seems to have added a seventh week as well, though neither number when you subtract Sunday and Saturday, gives the total of forty days? For only six and thirty days are included in these weeks.