StoveBolts I apologize for the length of this.
The thing is we have to go by the calendar and the numbering of days God established with Moses and Aaron as those days and dates never change, Exodus 12:1-6, like that of the Lunar Calendar I posted earlier. It was the Babylonians that gave names to the days of the week from Tuesday to Saturday after the five planetary bodies and after the sun and moon being Sunday and Monday. This custom was later adopted by the Romans.
The 10th day would have been a Monday that God established with Moses and Aaron as the day they were to take a lamb and keep it until the 14th being a Friday, even though the calendar changes year after year, but never God establishing the days or dates in the beginning with Moses and Aaron with the cycle of the moon's phases. It's like the Lunar Calendar I already posted as this would have been the one God established with the actual dates, but yet showing how those days and dates changed.
They shall keep their lambs until Friday the 14th and kill them in the evening as the lambs were sacrificed (prepared, day of preparation) on the 14th, but the Passover meal was not eaten until the 15th around midnight as God passed over Egypt. On the 14th in the even the lambs were slaughtered and they would have to let the blood drain out and collect it before they roasted them. It takes four to five hours to roast a lamb and since they were to eat this Passover meal around midnight before the Lord Passover this possibly started right before sunset and all finished before midnight being he 15th. This could be a reason why the Jews say the 14th ran into the 15th before midnight.
I can only assume it was right around sunset when they killed their lambs making it the early beginning of Saturday the 15th as this might be considered to be the 14th and 15th. Then they were instructed to spread the blood on the two side post and on the upper door post in preparation for God passing over Egypt (Passover at midnight on Saturday the 15th.
Being this was the Lord's Passover meal eaten with unleavened bread to be prepared and eaten that night with nothing left over before morning as they had to be prepared to flee in haste in the morning which would have made it sunrise on the 16th being a Sunday this was the memorial God said to keep throughout all the generations.
Go back to Matthew 26 beginning with vs. 17 as this is a reference to Exodus 12:6 which says the lamb's were killed in the evening of the 14th. This supper Jesus had with His disciples did not take place on the first night of Passover being the 15th. Jesus was tried, killed and buried before the day of Passover that began after sunset making it Saturday the 15th before Passover began.
In vs. 17-19 now the first day (not on the first day of the feast of unleavened bread) the disciples only asked Jesus where they should prepare the Passover meal. (On the first day you shall have a holy convocation, you shall do no customary work on it, Exodus 13:4-10; Leviticus 23:4-8; Numbers 28:16-25; Deuteronomy 16:1-8.) Jesus told them to go into the city and require of a place to keep the Passover meal as they only made ready the place for the Passover. Jesus told them that His time is at hand as He will keep the Passover at their house, but they did not understand that Jesus would be the Lamb slain before the Passover.
From vs. 20-75 it was only at that evening after they ate and song a hymn that with the disciples did Jesus come to the garden of Gethsemane to pray. Whether before sunset on Wednesday the 12 or right after sunset that would make it Thursday the 13 that Jesus would sit with the disciples in the upper room eating a meal with them, as being the last supper He would have with them, as He explains about His time being at hand and the events that would follow.
Matthew 27:1, 2 it was the morning, which I would think being very early in the morning on the 13th now being a Thursday, Jesus was bound and delivered to Pontius Pilate for sentencing. After being handed over to the very Jews who followed Jesus and heard Him teach they sentenced Him to be crucified. Jesus, by Roman law, was beaten and scourged then carried His cross beam up the half mile hill to Golgotha. At the third hour (3:00pm) he was nailed to the cross and died in the ninth hour (6:00PM) the day of the 13th being a Thursday and laid in the tomb until God raised Him from the tomb sometime around early morning making it Sunday the 16th.
Jesus was three days and three nights, (not a 24 hour period, but sunset to sunrise) in the tomb so He would have not been alive to celebrate the Passover with His disciples on the 15th being a Saturday.