The following is quoted from the Encyclopedia of Bible Difficulties.
Matthew 12:40: On which day of the week was Christ crucified?
Matthew 12:40 states: “For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth.” If the general tradition—that Christ was crucified on Friday of Holy Week, died at 3:00 P.M. (the “ninth hour” of the day), and then rose again from the dead on Sunday at dawn—is correct, how can it be said that Jesus was three days and three nights in the grave? He was interred about 6:00 P.M., according to Luke 23:54. (“And it was the day of preparation [hemera paraskeues] and the Sabbath was coming on [epephosken].”) This would mean that the period of interment was only from Friday night to Saturday night before the Resurrection on the dawn of Sunday; and it would also mean only one dawn-to-sunset day, namely Saturday, had passed. How do we get “three days and three nights” out of two nights and one day? Must not the actual day of crucifixion have been Thursday or even Wednesday?
It is perfectly true that a Friday Crucifixion will not yield three full twenty-four-hour days. But neither will a Thursday afternoon Crucifixion, nor a Wednesday afternoon Crucifixion either. This results from the fact that Jesus died at 3:00 P.M. and rose at or about 6:00 A.M. The only way you can come out with three twenty-four-hour days is if He rose at the same hour (three days later, of course) that He was crucified, namely, 3:00 P.M. Actually, however, He rose “on the third day” (1 Cor. 15:4). Obviously, if He rose on the third day, He could not already have been buried for three whole nights and three whole days. That would have required His resurrection to be at the beginning of the fourth day.
What, then, is the meaning of the expression in Matthew 12:40: “three days and three nights in the heart of the earth”? (NASB). This can only refer to three twenty-four-hour days in part or in whole. That is to say, Jesus expired at 3:00 P.M. near the close of Friday (according to the Hebrew method of reckoning each day as beginning at sundown), which would be one day. Then Friday 6:00 P.M. to Saturday 6:00 P.M. would be the second day, and Saturday 6:00 P.M. to Sunday 6:00 P.M. would constitute the third day—during which (i.e., Sunday 6:00 A.M. or a little before) Christ arose. Christ rested in hades (where paradise, or “Abraham’s Bosom,” still was, according to the indications of Luke 16:22-26; cf. Luke 23:43) for a portion of the three days: Friday, Saturday, and Sunday. The same would be true, or course, if the Evangelists had been reckoning according to the Roman method, from midnight to midnight.
Why then are three portions of day referred to in Matthew 12:40 as “three days and three nights”? The simple answer is that the only way “day” in the sense of dawn-to-dusk sunlight could be distinguished from the full twenty-four-hour cycle sense of “day” was to speak of the latter as “a night and a day” (i.e., an interval between 6:00 P.M. and 6:00 P.M. of the day following). In other words Friday as a twenty-four-hour unit began on Thursday 6:00 P.M. and lasted until Friday 6:00 P.M. Correspondingly, Sunday began at 6:00 P.M. Saturday, according to Hebrew reckoning (but 12:00 P.M. Saturday according to Roman reckoning). According to ancient parlance, then, when you wished to refer to three separate twenty-four-hour days, you said, “Three days and three nights”—even though only a portion of the first and third days might be involved.
A similar usage is apparent from the narrative in 1 Samuel 30:12, where “he had not eaten bread or drunk water for three days and three nights” is equated in v.13 with hayyom se losah (“three days ago”)—which could only mean “day before yesterday.” But if the Egyptian slave fell ill on the day before yesterday (with relationship to the day on which David found him), then he could not have remained without food or water for three entire twenty-four-hour days. We simply have to get used to slightly different ways of expressing time intervals. (“Similarly the Feast of Pentecost was originally called the “Feast of Weeks” because it fell on the forty-ninth day after the offering of the wave sheaf on the first day of the Feast of Unleavened Bread. Yet it was known actually as the Fiftieth Day— Pentecoste in Greek.)