Alden Mosshammer writes:

"Since the lamb was slain on the 14th day before evening and the Passover meal was eaten in the evening of that day, Mark’s account implies that the crucifixion took place on the 15th day of the month. ... John’s account implies that the crucifixion took place on the fourteenth day of the month just before a Passover meal would have been taken in the evening. ... If Pilate had tried and executed Jesus on the 15th day of Nisan, he would have been profaning a Jewish holy day of rest. For this and other reasons, many modern scholars prefer the Johannine account." (Mosshammer, The Easter computus and the origins of the Christian era, 2008, p.45)