But He answered and said to them, “An evil and adulterous generation seeks after a sign, and no sign will be given to it except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth.”
Listen to the "Verse by Verse" episode covering this scripture.
In our Fundamental Beliefs “8. Three Days and Three Nights,” we state, “Jesus Christ was raised from the dead by the Father after His body lay for three days and three nights in the grave. The length of time that He was in the grave was the only sign He gave to prove He was the Messiah.”
Jesus had done many miracles, but still the Pharisees sought “how they might destroy Him” (Matthew 12:14). So Jesus didn’t offer them more miracles as a sign, only this reference to the length of time He would be in the grave.
Strangely, though, most Christians today do not believe that Christ was literally three days and three nights in the grave as He promised, since it is not possible to count three days and three nights between Good Friday afternoon and Easter Sunday morning.
For a detailed explanation of the biblical chronology of Christ’s crucifixion and resurrection, including a helpful chart, see “Sign of Jonah: Did Jesus Die Good Friday, Rise on Easter?”