Luke 11:30
. As Jonah was a sign to the Ninevites. He declares that he will be a sign to them, as Jonah was to the inhabitants of Nineveh. But the word sign is not taken in its ordinary sense, as pointing out something, but as denoting what is widely removed from the ordinary course of nature. In this sense Jonah’s mission was miraculous, when he was brought out of the belly of the fish, as if from the grave, to call the Ninevites to repentance. Three days and three nights This is in accordance with a well-known figure of speech. (166) As the night is an appendage to the day, or rather, as the day consists of two parts, light and darkness, he expresses a day by a day and a night, and where there was half a day, he puts down a whole day.
(166) “ Quant aux trois nuits, il y a ici (cornme on scait bien) une figure que les Grecs et Latins appellent Synecdoche;” — “as to the three nights, there is here (as is well known) a figure which the Greek and Latin writers call Synecdoche.”