This discrepancy is conceded by Scribner’s “Bible Dictionary.” It says:
“It is the Last Supper which the Synoptics appear to fix by identifying it with the Passover. They say expressly that on the morning of the ‘first day of unleavened bread, when they sacrificed the passover’ ([Mk. xiv, 12]), the disciples asked where the Passover was to be eaten. This would be on the morning of Nisan 14. In the evening, which from twilight onwards would belong to Nisan 15, would follow the Last Supper, and on the next afternoon (still, on the Jewish reckoning, Nisan 15) the crucifixion. St. John, on the other hand, by a number of clear indications ([John xiii, 1], [xviii, 28], [xix, 14], [31]) implies that the Last Supper was eaten before the time of the regular Passover, and that the Lord suffered on the afternoon of Nisan 14, about the time of the slaying of the Paschal lamb. We are thus left with a conflict of testimony.”
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On what day of the week was he crucified?
Synoptics: On Friday.
John: On Thursday.
The Synoptics agree that he was crucified on the day following the Preparation, that is, on the day of the Passover, and the day preceding the Sabbath. As the Jewish Sabbath fell on Saturday, he was, therefore, crucified on Friday.
John repeatedly declares that his trial and crucifixion occurred on “the preparation of the passover.” If the Passover occurred on Friday, as the Synoptics state, he was crucified on the preceding day, or Thursday. It is claimed by some, though the claim is disputed, that the Synoptics are in error, that the Passover was never held on Friday.