Objection 1. It would seem that Christ was not in the tomb during only one day and two nights; because He said (Matthew 12:40): "As Jonas was in the whale's belly three days and three nights: so shall the Son of man be in the heart of the earth three days and three nights." But He was in the heart of the earth while He was in the grave. Therefore He was not in the tomb for only one day and two nights.
Objection 2. Gregory says in a Paschal Homily (Hom. xxi): "As Samson carried off the gates of Gaza during the night, even so Christ rose in the night, taking away the gates of hell." But after rising He was not in the tomb. Therefore He was not two whole nights in the grave.
Objection 3. Further, light prevailed over darkness by Christ's death. But night belongs to darkness, and day to light. Therefore it was more fitting for Christ's body to be in the tomb for two days and a night, rather than conversely.
On the contrary, Augustine says (De Trin. iv): "There were thirty-six hours from the evening of His burial to the dawn of the resurrection, that is, a whole night with a whole day, and a whole night."
I answer that, The very time during which Christ remained in the tomb shows forth the effect of His death. For it was said above (Question 50, Article 6) that by Christ's death we were delivered from a twofold death, namely, from the death of the soul and of the body: and this is signified by the two nights during which He remained in the tomb. But since His death did not come of sin, but was endured from charity, it has not the semblance of night, but of day: consequently it is denoted by the whole day during which Christ was in the sepulchre. And so it was fitting for Christ to be in the sepulchre during one day and two nights.
Reply to Objection 1. Augustine says (De Consens. Evang. iii): "Some men, ignorant of Scriptural language, wished to compute as night those three hours, from the sixth to the ninth hour, during which the sun was darkened, and as day those other three hours during which it was restored to the earth, that is, from the ninth hour until its setting: for the coming night of the Sabbath follows, and if this be reckoned with its day, there will be already two nights and two days. Now after the Sabbath there follows the night of the first day of the Sabbath, that is, of the dawning Sunday, on which the Lord rose. Even so, the reckoning of the three days and three nights will not stand. It remains then to find the solution in the customary usage of speech of the Scriptures, whereby the whole is understood from the part": so that we are able to take a day and a night as one natural day. And so the first day is computed from its ending, during which Christ died and was buried on the Friday; while the second. day is an entire day with twenty-four hours of night and day; while the night following belongs to the third day. "For as the primitive days were computed from light to night on account of man's future fall, so these days are computed from the darkness to the daylight on account of man's restoration" (De Trin. iv).
Reply to Objection 2. As Augustine says (De Trin. iv; cf. De Consens. Evang. iii), Christ rose with the dawn, when light appears in part, and still some part of the darkness of the night remains. Hence it is said of the women that "when it was yet dark" they came "to the sepulchre" (John 20:1). Therefore, in consequence of this darkness, Gregory says (Hom. xxi) that Christ rose in the middle of the night, not that night is divided into two equal parts, but during the night itself: for the expression "early" can be taken as partly night and partly day, from its fittingness with both.
Reply to Objection 3. The light prevailed so far in Christ's death (which is denoted by the one day) that it dispelled the darkness of the two nights, that is, of our twofold death, as stated above.
Source: Summa Theologica (New Advent)