Exodus 12:3 "Speak to all the congregation of Israel, saying: ‘On the tenth day of this month (Abib / Nisan) every man shall take for himself a lamb, according to the house of his father, a lamb for a household. 4 ‘And if the household is too small for the lamb, let him and his neighbor next to his house take it according to the number of the persons; according to each man’s need you shall make your count for the lamb. 5 ‘Your lamb shall be without blemish, a male of the first year. You may take it from the sheep or from the goats. 6 ‘Now you shall keep it until the fourteenth day of the same month. Then the whole assembly of the congregation of Israel shall kill it at twilight. 7 ‘And they shall take some of the blood and put it on the two doorposts and on the lintel of the houses where they eat it. 8 ‘Then they shall eat the flesh on that night; roasted in fire, with unleavened bread and with bitter herbs they shall eat it. 9 ‘Do not eat it raw, nor boiled at all with water, but roasted in fire—its head with its legs and its entrails. 10 ‘You shall let none of it remain until morning, and what remains of it until morning you shall burn with fire. 14 ‘So this day shall be to you a memorial; and you shall keep it as a feast to the LORD throughout your generations. You shall keep it as a feast by an everlasting ordinance. 15 ‘Seven days you shall eat unleavened bread. On the first day you shall remove leaven from your houses. For whoever eats leavened bread from the first day until the seventh day, that person shall be cut off from Israel. 16 ‘On the first day there shall be a holy convocation, and on the seventh day there shall be a holy convocation for you. No manner of work shall be done on them; but that which everyone must eat—that only may be prepared by you. 17 ‘So you shall observe the Feast of Unleavened Bread, for on this same day I will have brought your armies out of the land of Egypt. Therefore you shall observe this day throughout your generations as an everlasting ordinance. 18 ‘In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening.
Leviticus 23:5 ‘On the fourteenth day of the first month at twilight is the LORD’S Passover.
Numbers 9:5 And they kept the Passover on the fourteenth day of the first month, at twilight, in the Wilderness of Sinai; according to all that the LORD commanded Moses, so the children of Israel did.
Joshua 5:10 Now the children of Israel camped in Gilgal, and kept the Passover on the fourteenth day of the month at twilight on the plains of Jericho.
The instructions are clear - the lamb must be slain on the 14th day of Nisan at dusk, and eaten in the beginning of the 15th day. As I am sure you are well aware that the difficulties arise as follows:
Certainly the death of the Passover lamb foreshadowed the death of Jesus.
1 Corinthians 5:7 Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us.
So as you say, it seems reasonable that Jesus would die on the 14th of Nisan, but we can only insist on it by disregarding the seemlingly clear statements in the synoptic gospels. It's a bit of a conundrum.
I hope you find this helpful.