Matthew 12:40 - Wesley's Explanatory Notes

Three days and three nights — It was customary with the eastern nations to reckon any part of a natural day of twenty — four hours, for the whole day. Accordingly they used to say a thing was done after three or seven days, if it was done on the third or seventh day, from that which was last mentioned. Instances of this may be seen, 1 Kings 20:29; and in many other places. And as the Hebrews had no word to express a natural day, they used night and day, or day and night for it. So that to say a thing happened after three days and three nights, was with them the very same, as to say, it happened after three days, or on the third day. See Esther 4:16; Esther 5:1; Genesis 7:4, Genesis 7:12; Exodus 24:18; Exodus 34:28. Jonah 2:1.