Modern Jewish calendars list each of the seven days of the festival as "Passover". The feast of Unleavened Bread therefore was not a separate event or meal that followed "Passover" but was this same "feast." Passover extended from Nisan 15 which was a "holy convention" or special Sabbath day (Seder) through to Nisan 21, which was also a "holy convention" or special Sabbath day. The actual Passover occurred in the evening and the memorial of that celebration that was repeated year after year copied the actual event. This was not a 24-hour celebration but is the start of the Festival of Unfermented Cakes.
And on the first day there is to take place for YOU a holy convention, and on the seventh day a holy convention. No work is to be done on them. Only what every soul needs to eat, that alone may be done for YOU. (Exodus 12:16)
Now the festival of the unfermented cakes, the so-called Passover, was getting near. (Luke 22:1, NWT)
Now the feast of unleavened bread drew nigh, which is called the Passover. (Luke 22:1, KJV)
If the Passover were to be eaten on the evening 13/14, then the entire daytime of Nisan 14 would be excluded from the upcoming festival of the unfermented cakes. That festival starts on Nisan 15 as the Scriptures clearly state.
In the first month, on the fourteenth day of the month, between the two evenings is the passover to Jehovah. "‘And on the fifteenth day of this month is the festival of unfermented cakes to Jehovah. Seven days YOU should eat unfermented cakes.’" (Leviticus 23:5-6)
The Passover lamb is to be slaughtered and prepared (quickly roasted) to be eaten between the two evenings of the 14th. Typically that is the time after the sun sets to the time when the after—glow fades and it is completely dark.
"Now the sons of Israel should prepare the passover sacrifice at its appointed time. On the fourteenth day in this month between the two evenings YOU should prepare it at its appointed time. According to all its statutes and all its regular procedures YOU should prepare it." (Numbers 9:2-3)
The lamb had to be kept safe until the 14th, which means that it could not be slaughtered on the 13th. The lamb had to be slaughtered on Nisan 14 before the date changed to Nisan 15. It had to be roasted quickly and eaten hot that night (actually Nisan 14/15 evening).
And it must continue under safeguard by YOU until the fourteenth day of this month, and the whole congregation of the assembly of Israel must slaughter it between the two evenings. (Exodus 12:6)
"‘And they must eat the flesh on this night. They should eat it roasted with fire and with unfermented cakes along with bitter greens.’" (Exodus 12:8)
According to the Scriptural account of the incidents surrounding the time immediately following the Israelites release from Egypt, the Israelites left on the fifteenth of the month directly after the Passover. The splashing of the doorposts with lamb’s blood and the hasty preparation of meal was completed before that evening. The rest of the night was set aside for the Israelites to eat together as families and sing songs waiting for their deliverance to come that night by means of Jehovah’s angel and Pharaoh’s release announcement. The Passover preparation must occurred on the fourteenth (the lamb was kept safe until the 14th) and eaten during the evening of 14th or beginning of 15th, after nightfall. The meal could be eaten later. It was the preparation that carried a specific timetable. During the night of the 15th in the early morning hours, Jehovah’s angel did its job and their release was issued. Prepared as commanded, that morning of the 15th, Israel picked up their belongings and left.
And they proceeded to pull away from Ram'e·ses in the first month, on the fifteenth day of the first month. Directly the day after the passover the sons of Israel went out with uplifted hand before the eyes of all the Egyptians. (Numbers 33:3)
The gospel accounts of Jesus’ last days also support the late date (evening 14th /15th) Passover.
Now on the first day of unfermented cakes, when they customarily sacrificed the passover [victim], his disciples said to him: "Where do you want us to go and prepare for you to eat the passover?" (Mark 14:12)
The day of the unfermented cakes now arrived, on which the passover [victim] must be sacrificed; (Luke 22:7 ) [That day would be Nisan 14. (Exodus 12:6)]