In Matthew 12, Jesus makes a prophecy concerning his own burial.
Then some of the scribes and Pharisees said to Him, “Teacher, we want to see a sign from You.” But He answered and said to them, “An evil and adulterous generation craves for asign; and yet nosign will be given to it but the sign of Jonah the prophet; for just as JONAH WAS THREE DAYS AND THREE NIGHTS IN THE BELLY OF THE SEA MONSTER, so will the Son of Man be three days and three nights in the heart of the earth.
This same story also appears in Luke 11 in more general terms, and there’s a further reference to the Sign of Jonah in Matthew 16, but Matthew 12 is unique in specifying exactly three days and three nights. And it’s a problem, because according to the Gospels, this prophecy got the duration wrong.
According to the Gospels, Jesus was crucified, died, and was buried on the day before the Sabbath. Thus, the beginning of the “three days and three nights” would be around sunset Friday evening. From Friday sunset to Saturday sunset is one day and one night. From Saturday sunset to Sunday sunset is also one day and one night. But oops! Jesus is supposed to have arisen around sunrise on Sunday morning. So that’s two days and two nights, but take away one of the days, because the resurrection allegedly happened before the second day started. That means instead of three days and three nights, the Son of Man was only two nights and one day in the heart of the earth.
Christians sometimes get around this by explaining that the Jewish day begins at sunset, which is an interesting bit of trivia, but doesn’t really solve the problem. If he was in the tomb by a few minutes before sunset on Friday, ok, count those few minutes as “one day” because it was Friday. Then the Sabbath begins with the sunset, and ends with the following sunset, which adds one day and one night. Then the third day (by Jewish reckoning) begins at sunset, and adds one more night, but oops again, it’s still only a day and a half later, and counting the last few minutes of Friday as a full “day” only brings us to two days and two nights, so Jesus is still wrong.
If you reeeeally want to say that Jesus was in the tomb 3 days and 3 nights, you have to lie a little bit and count the same night as being two different nights. Thus, you can go back to the Roman system of counting the days from midnight to midnight, but also mix it up a little bit with the Jewish system of counting days from sunset to sunset, and say, “Ok, he was in the tomb for the last few minutes before sunset on Friday, so that’s one ‘day,” then he was in the tomb from sunset Friday to midnight Friday, so that’s one ‘night.’ Then from midnight (between Friday and Saturday) until sunrise Saturday morning is another ‘night,’ (even though it’s really the same night as before midnight) Saturday sunrise to sunset is another day, Saturday sunset until midnight is another ‘night’, and then midnight until the following sunrise is another night. Then if Jesus rises from the dead a little after sunrise, we have a few minutes of Sunday morning that we can call another ‘day.’ So by this system, we have 3 days and—one, two, three, oops! four nights. Darn.”
In the end you have to use the last method, except you count one of the nights as two ‘nights’ and two of the ‘nights’ as being one night because shut up that’s why. In other words, you just have to lie. Matthew says Jesus predicted that he would be “in the heart of the earth” for “three days and three nights,” and if you’re a believer you simply have to ignore the fact that this prediction never actually because true. For bonus points, you can even claim that the Sign of Jonah is one of the fulfilled prophecies that proves Jesus was God, despite the fact that it was wrong.