For that the three days were not three full and entire days, Scripture witnesses; the first day is reckoned because the latter end of it comes in; and the third day is likewise reckoned, because the first part of it is included; while the day between, that is the second day, appears in all its twenty-four hours, twelve of the night and twelve of the day. For the succeeding night up to the dawn when the Lord’s resurrection was made known, belongs to the third day. For as the first days of creation were, because of man’s coming fall, computed from morning to night; so these days are because of man’s restoration computed from night to morning.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: De Trin., iv. 6