“‘For just as Jonah was three days and three nights in the belly of the sea monster, so shall the Son of Man be three days and three nights in the heart of the earth.'” (Matthew 12:40)
If there’s something you don’t understand in the Bible (including a possible discrepancy), remember, it’s not the Bible’s problem. It’s your own understanding’s problem.
Jesus was crucified beginning at 9 a.m. on a Friday. (Mark 15:25) He died at 3 p.m. (Mark 15:34-37) He was buried in a tomb not too long after that. Then came Saturday. And on the next day, Sunday, in the morning He rose from the dead. (Mark 16:2-6) The question arises: How do you get three days and three nights out of that? A portion of Friday plus Saturday and a portion of Sunday. You barely get one whole day, a couple of nights (Friday and Saturday) and portions of two other days (Friday and Sunday). Right?
One answer has to do with the Jewish thinking on what constituted a “day.” Any part of a day was considered as a whole day (including the night). And so we have “parts” of Friday and Sunday plus Saturday equals three days (and three nights). Follow me?
Enter the next problem. Jesus couldn’t have been more clear about the “three nights” in His statements. If He would have left it at “three days” without the nights, the Jewish thinking just mentioned is confirmed because it’s based on what constituted a “day.” (Oh, Jesus, why did You have to reference the “nights,” too? You messed us all up.)
Jesus Christ’s Last Supper ‘was on a Wednesday’: http://www.bbc.co.uk/news/world-13114124