Mark says this is on the first day of the Feast and the day the Jews sacrificed the Passover lamb. The lamb was killed on the fourteenth day of the Hebrew month of Nisan, making Jesus' death the following day on the fifteenth, Passover. Mark states that the lamb was sacrificed on the first day of the feast, having the dinner on the same day as the sacrifice of the lamb. This would be correct from our modern notion of a day beginning at Midnight, but the Jewish day began at sundown and so the Passover dinner from their point of view occurred the day after the sacrifice. Either Mark is using a non-Jewish reckoning of time or is using his method of double time chronology, such as 1:32, where two temporally separated events are sandwiched together. The other synoptic gospels agree with Mark on this chronology. John 19:14 however has Jesus' death occur during the slaughter of the Passover lamb, making his death the fourteenth and thus the Last Supper on the thirteenth and so not the Passover meal.