Matthew 12:40 is a reference by Jesus to His forthcoming stay in the tomb and this is what it says, “For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth.”
Every Easter season this question comes up and based on this verse someone will argue that the crucifixion took place on Wednesday or someone will argue that the resurrection took place on Monday.
So, let me point out some things to help you properly understand the meaning…
By Hebrew reckoning, any part of a day is considered that day. So the three days of Jesus being in the tomb are part of the first day, all of the second day, and part of the third day. A day was also counted as starting at sundown and ending at sundown.
Genesis 1:5 says, “And the evening and the morning were the first day.” We understand the start and the ending of day as midnight, but the Hebrews recognized a day as being from sunset to sunset (Leviticus 23:32, Mark 1:32).
Jesus died about 3:00 PM on Friday and rose before dawn on Sunday. That would be about 3-4 hours on Friday before sundown (Preparation Day), all 24-hours on Saturday (Sabbath), and he would arise sometime after sundown on what we would call Saturday night (the first day of the week). On the first day of the week there would have been about 12 hours into the day (Sunday) when Mary went to the tomb and thought he was the gardener.
An accurate understanding of Matthew 12:40 doesn’t require us to assume it’s referencing a 72-hour period. Jonah likely was not not in a fish for a literal minimum of 72-hours and Jesus was not in the tomb for a literal minimum of 72-hours, either.
We actually do a similar expression in our English and recognize a part of a day as a full day in our speech. An example would be someone might say “I worked all day” but it doesn’t mean they worked for a literal minimum of 24-hours.
I think the lesson here is to consider the historical and cultural background meanings of a text before reading into a text our 2022 understanding of things.