About the crucifixion contradiction.

I'm currently discussing the (apparent?)contradiction regarding the time when Jezus was crucified.Mark, Matthew and Luke are in agreement that the crucifixion took place on the first day of Pascha and so "preparation day" refers to the day before the Sabbath. So the Seder meal had allready taken place. John on the other hand puts the crucifixion on the day before Passcha and uses the words "preparation day" to refer to the day before Passover in my opinion.However my opponent argues that John's "preparation day" is a reference to the Feast of Unleavened Bread. According to him the preparation day for Passover can both refer to the Passover Celebration and the Feast of the Unleavened Bread this way resolving the contradiction.Am I correct that John 18:28 negates his argument because it states that the Passover meal is scheduled for that evening (following the crucifixion)? And isnt my opponent just adding to the text when he states that the preparation day of Passover can refer to both Passcha and the Feast of the Unleavend bread?Either way I'm a bit confused. For instance when does Passover exactly starts -Nissan 14 or 15? I have conflicting sources. Does Passover start with the Seder or with the lamb slaughtering?Any comments would be highly appreciated,Dino