For as Jonah was three days and three nights in the belly of the great fish; so shall the Son of man be three days and three nights in the heart of the earth.
All Commentaries on Matthew 12:40 Go To Matthew 12
Theophylact of Ochrid
AD 1107
He calls them an evil generation as they were deceiving tempters, and adulterous, because they had abandoned God and followed after demons. He calls His Resurrection a sign as it is marvelous beyond belief. For having descended into the heart of the earth, the nethermost part, which is hades, He arose on the third day. The three days and three nights you must understand as spoken of in part and not in their entirety. For He died on Friday, which is one day. He was dead on Saturday — behold, the second day. And the night of Sunday held Him still dead. The three days and nights, then, are counted by parts, in just the same way as we often count them ourselves.