Fragments of St. Anatolius of Laodicea

And, therefore, in this concurrence of the sun and moon, the Paschal festival is not to be celebrated, because as long as they are found in this course the power of darkness is not overcome; and as long as equality between light and darkness endures, and is not diminished by the light, it is shown that the Paschal festival is not to be celebrated. Accordingly, it is enjoined that that festival be kept after the equinox, because the moon of the fourteenth, if before the equinox or at the equinox, does not fill the whole night. But after the equinox, the moon of the fourteenth, with one day being added because of the passing of the equinox, although it does not extend to the true light, that is, the rising of the sun and the beginning of day, will nevertheless leave no darkness behind it. And, in accordance with this, Moses is charged by the Lord to keep seven days of unleavened bread for the celebration of the Passover, that in them no power of darkness should be found to surpass the light. And although the outset of four nights begins to be dark, that is, the 17th and 18th and 19th and 20th, yet the moon of the 20th, which rises before that, does not permit the darkness to extend on even to midnight.