38 Then some of the scribes and Pharisees answered and said to Him, “Teacher, we want to see a sign from You.” 39 But He answered and said to them, “An evil and adulterous generation eagerly seeks for a sign; and yet no sign will be given to it but the sign of Jonah the prophet; 40 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. 41 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 12:38-41 (LSB)
Today is Good Friday. Passover was a feast instituted by God to be held on Nisan 14 every year. Also, the Jewish day always began at sunset. Therefore, Nisan 14 began at sunset on Thursday and ran until sunset on Friday. At sunset Nisan 15 would begin. The next festival or feast after Passover was Unleavened Bread. This feast was eaten in the afternoon following the Seder, which was the Passover meal. Notice that this feast was held prior to sunset on Nisan 15 so it would still be Nisan 14.
Now, with this in mind let us look at something that is quite troubling to many Christians. In Matthew 12:40 Jesus made this statement, “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.” However, using our 21st Century methods of calculating time and our Western mindsets we struggle lining up what Jesus said here with the timing of His crucifixion, burial and resurrection.