A wicked and adulterous generation seeketh after a sign: and a sign shall not be given it, but the sign of Jonas the prophet. And he left them, and went away.
 Now the Lord prepared a great fish to swallow up Jonas: and Jonas was in the belly of the fish three days and three nights.
Saint Jerome writes about this passage:
‘and Jonah was in the belly of the whale for three days and three nights’. The Lord shows in the Gospel the symbolism of this passage, and it is superfluous to say in the same terms or even in other terms what he who has suffered has already said. But we ask ourselves this: how was he three days and three nights in the belly of the earth. Some scholars take the view according to paraskeuen (day of preparation for the Sabbath), because of the solar eclipse from the sixth to the ninth hour when night followed day, this would be two days and nights, and adding the Sabbath, believe that we should count this as three days and three nights. But I prefer to understand this by reason of synecdoche, seeing the whole as a part: where he is dead in paraskeuen, let us count one day and one night; two with the Sabbath; the third night which arises from the day of the Lord, let us take that as the beginning of the next day, for, in Genesis the night is not of the preceding day, but of the following day, that is to say the beginning of the next day, not the end of the previous. To understand this better I will say it more simply: if a man leaves his house at nine and the next day he arrives at his other house at three. And if I say that he has been two days in travelling, I will not be reprimanded as a liar, because he has not used all the hours of two days, but only a part for his journey. Nonetheless this seems to me to be the interpretation. If someone does not agree with this, and he can explain the meaning in a clearer way, then we should follow his interpretation.