The Jewish calendar must have been at least as central to the lives of Jewish people at the time of Christ as our calendar is to us today. For example, on Nisan 1, in the spring of each year, Jews would have marked the start of their religious year (their civil year, used for official purposes, started in the autumn). In the first two weeks of Nisan, Jewish people would have travelled from many countries to Jerusalem to celebrate the great Feast of Passover. On Nisan 10 they would have selected a Passover lamb for sacrifice at the temple in Jerusalem on Nisan 14, as instructed in the book of Exodus. On Nisan 15 they would have eaten their Passover meal and remembered the events of the Exodus. On Nisan 16 they might have watched the barley being harvested in the fields, after seeing the priests waving the first sheaves of barley in the temple earlier that day, as specified in the book of Leviticus, and so on.

The Jewish calendar must have been so central to the lives of Jewish people in the first century ad, for both religious and agricultural reasons, that to my mind it is simply not credible that the traditions handed down by eye-witnesses to the gospel writers were confused about whether Jesus died on Nisan 14 or 15, particularly because these two days, when the Passover lambs were slain, roasted and eaten, were the most important days in the whole year in the Jewish calendar.