John Wesley on Matthew 12:40

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, 1Kings 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; 5:1; Gen 7:4,12; Exod 24:18; 34:28. Jonah 2:1.