"But [Jesus] answered and said to them, "An evil and adulterous generation craves for a sign; and yet no sign will be given to it but the sign of Jonah the prophet; for just as Jonah was three days and three nights in the belly of the sea monster, so will the Son of Man be three days and three nights in the heart of the earth. The men of Nineveh will stand up with this generation at the judgment, and will condemn it because they repented at the preaching of Jonah; and behold, something greater than Jonah is here."" Matthew 12:39-41
People believe that the "sign of Jonah" was the three days and three nights. But take notice of the fact that the parallel passage in Luke says nothing about a time period. Jesus' emphasis is on the way His people rejected His ministry, preaching, and prophecy while the Ninevites received the preaching of Jonah and repented.
"As the crowds were increasing, [Jesus] began to say, "This generation is a wicked generation; it seeks for a sign, and yet no sign will be given to it but the sign of Jonah. For just as Jonah became a sign to the Ninevites, so will the Son of Man be to this generation. ... The men of Nineveh will stand up with this generation at the judgment and condemn it, because they repented at the preaching of Jonah; and behold, something greater than Jonah is here."" Luke 11:29-30, 32
After Jonah had come out of the sea monster, it took him three days to reach Nineveh. He then entered the city one-half day's journey and preached for 40 days that the city would be destroyed (Jonah 3:3-4).
We can see this same sequence of three and one-half followed by 40 elsewhere in Scripture. Elijah hid himself for three and one-half years during the famine, and then fled for 40 days from Jezebel.
Now, watch this!
After Jesus was anointed by His baptism, He preached to the Jews for three and one-half years, warning that in one generation—40 years—the city and the temple would be destroyed (Matt. 12:41). Because the nation of Israel did not listen and repent, it was destroyed. Only a small percentage of the Jews accepted Him and were ready. You can take it to the bank that this will again happen to the Church at the time of Jesus' second coming.
When were Jerusalem and the temple destroyed? In A.D. 70. Therefore, if the pattern holds true—three and one-half followed by 40, if we subtract 40 from 70 we get 30. Once again, the most probable date for the crucifixion of Christ Jesus: Friday, April 7, A.D. 30 at 3:00PM.