[Note: It is without question that Mark and Luke both state that the Passover was on a day of unleavened bread. It was an historical view point that had nothing to do with Jewish tradition (Mk. 14; Lk. 22). Also, the day the firstborn were sanctified was also a day of unleavened bread (Ex. 13), and the Passover (slaying of the lamb and sprinkling of the blood) fell on the 14th day of the month Nissan (Lev. 23).
However a conflict arises as a result of two differing schools of thought, which are generally represented by the Sadducees who believed the Passover was between the two evenings beginning the 14th day of the month, and the Pharisees who believed the Passover was in the afternoon of the 14th between the two evenings of the 14th day and 15th day of the month of Nissan.
The problem is resolved by realizing Scripture places the Passover on the 14th of Nisan (Lev. 23), and the slaying of the lamb and sprinkling of the blood was the same event that sanctified the firstborn, which fell on a day of unleavened bread. This means the slaying of the lamb and the sprinkling of the blood occurred on the 14th of Nisan (a day of unleavened bread), which was on the same day the firstborn were slain at midnight.
Consequently, then, the Passover, the sanctification of the firstborn, and the killing of the firstborn occurred on the same day—the 14th of Nisan. Therefore the sacrifice and sprinkling of the blood (sanctification) had to occur before the death angel passed over the firstborn at midnight on that same day, which places the Passover between the two evenings beginning the 14th of Nissan.]
[Note: To say the sanctification occurred on the Festival of Unleavened Bread leads to the incorrect conclusion the firstborn were sanctified twice on two consecutive days respectively, and it also assumes the Passover took place on the 15th of Nissan. Also, if one argues that the phrase “remember this day” is a general expression it still includes the Passover as a day of unleavened bread because it would include the day of sanctification.]
[Note: The distinction of the observance called the “Last Great Day” was seen as inclusive to the Feast of Tabernacles, and accounted as the “eighth day” because it was also a day to “tabernacle,” and the Passover memorial was associated to the Festival of Unleavened Bread by reason of it being a day on which unleavened bread was to be eaten by the congregation of Israel (Acts 12:3; 20:6). This is understood in that there were “three times in a year” to be observed, and so it is implied by those who give a monetary offering on the Last Great Day that it is considered inclusiveness to the Festival of Tabernacles as an “eighth day” of the feast (Deut. 16:16).]