9 We begin the fast of forty days on the sixth day of Phamenoth; and having passed through that properly, with fasting and prayers, we may be able to attain to the holy day. For he who neglects to observe the fast of forty days, as one who rashly and impurely treads on holy things, cannot celebrate the Easter festival. Further, let us put one another in remembrance, and stimulate one another not to be negligent, and especially that we should fast those days, so that fasts may receive us in succession, and we may rightly bring the feast to a close.
Source: Letters (New Advent)