The Conferences Of John Cassian by John Cassian
Chapter XXVII. Why Lent is kept by very many with a different number of days.
Further, as for what you say; viz., that in some countries Lent is kept in different ways, i.e., for six or seven weeks, it is but one system and the same manner of the fast that is preserved by the different observance of the weeks. For those who think one ought to fast also on the Saturday, have determined on the observance of six weeks. They therefore fast for six days out of the seven, and this being six times repeated makes up the six and thirty days. It is therefore, as we said, but one system and the same manner of the fast, although there seems to be a difference in the number of the weeks.