And in the fourteenth day of the first month is the passover of the LORD. And in the fifteenth day of this month is the feast: seven days shall unleavened bread be eaten. In the first day shall be an holy convocation; ye shall do no manner of servile work therein: But ye shall offer a sacrifice made by fire for a burnt offering unto the LORD; two young bullocks, and one ram, and seven lambs of the first year: they shall be unto you without blemish: And their meat offering shall be of flour mingled with oil: three tenth deals shall ye offer for a bullock, and two tenth deals for a ram; A several tenth deal shalt thou offer for every lamb, throughout the seven lambs: And one goat for a sin offering, to make an atonement for you. Ye shall offer these beside the burnt offering in the morning, which is for a continual burnt offering. After this manner ye shall offer daily, throughout the seven days, the meat of the sacrifice made by fire, of a sweet savour unto the LORD: it shall be offered beside the continual burnt offering, and his drink offering. And on the seventh day ye shall have an holy convocation; ye shall do no servile work. (Numbers 28:16-25)
This passage tells us several things about the Passover which are helpful in understanding the events of the crucifixion. First, we note in this passage that the Passover proper is on the fourteenth day of the month Nisan. However, this day of the Passover is then followed by the feast of the Passover which is a seven day period of sacrifices and feasting beginning on the fifteenth of Nisan. We can also see in this passage that only unleavened bread was to be eaten during the seven days of the Passover feast.
When we compare Numbers 28 with Exodus 12, we learn further that the first Passover meal occurs on the evening of the fourteenth, and this first meal is also supposed to be eaten with unleavened bread. Thus the fourteenth is sometimes referred to as the first day of unleavened bread. Exodus 12:19 also tells us that, during the seven days of feasting beginning on the fifteenth of Nisan, the Jews were not only to eat unleavened bread, but they were also to have no leaven anywhere in their homes. Thus the seven days of feasting beginning on the fifteenth are sometimes referred to as the days of unleavened bread. The fact that the Jews were not allowed to have leaven in their houses during the week of feast days also explains why the fourteenth was referred to as the day of preparation. The evening of the fourteenth was spent in celebration of the Passover proper with a meal of lamb, bitter herbs and unleavened bread while the following day of the fourteenth was spent removing all leaven from the home in preparation for the Passover week.
Now when we return to the accounts in John and Mark, we can see that both accounts are accurate. Jesus was crucified at 9:00 AM on Thursday, the fourteenth of Nisan. The Passover meal would have been eaten during the evening of the fourteenth which, according to the Jewish method of counting time was the night before Jesus was crucified. The night following the crucifixion was the evening of the fifteenth of Nisan which was the first day of the seven days of feasting which made up the Passover week.
Therefore, when John mentions in John 18:28 that the Jewish leaders did not want to defile themselves because they wanted to eat the Passover, he was referring to their desire to participate in the seven days of feasting which would begin that evening. When John writes in John 19:14 that it was the day of preparation, he was referring to the preparations conducted on the fourteenth in order to remove all traces of leaven from the homes of the Jews. And when Mark mentions in Mark 14:12 that the Last Supper was on the first day of unleavened bread, the day when the Passover was killed, he was referring to the evening of the fourteenth of Nisan.
Thus the comparison of the crucifixion account with the instructions regarding the Passover that are found in the Old Testament reveals to us that both John and Mark are correct. Jesus died during the day of preparation which was after the evening of the Passover proper but before the first feast of the Passover week. There is no contradiction between the two accounts. They are simply describing an event which the average, gentile American knows practically nothing about.
If you would like a much more detailed explanation of the crucifixion week, you can click here to read an excellent collection of research by Lori Eldridge answering the question “What Day of the Week Was Christ Crucified?”