This verse is usually interpreted as a prophetic word by Jesus about Himself:
Matthew 12:40: 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.—King James Version
Jesus was crucified and buried on Friday night. On Sunday morning he was discovered by Mary as a resurrected One, which means that only two nights had passed: one from Friday to Saturday, and the other one from Saturday to Sunday.
Can anyone please explain to me, in simple terms, how is it three days and three nights?