The Passover is the 14th of Nisan, the death angel passed over the Israelites at midnight when they were in Egypt. Christ died on the Passover day at around 3 pm in the afternoon. The Passover is the preparation day, the lambs were killed and prepared in that day in Old Testament times. Today we prepare our homes, deleaven, and prepare spiritually and physically on that day for the service that night. The Passover is prepared on the 14th and eaten after sunset that evening the beginning of the 15th. It is eaten at the start of the first day of Unleavened Bread.

2Ch 30:15  Then they slaughtered the Passover lambs on the fourteenth day of the second month. The priests and the Levites were ashamed, and sanctified themselves, and brought the burnt offerings to the house of the LORD.

Joh 2:23  Now when He was in Jerusalem at the Passover, during the feast, many believed in His name when they saw the signs which He did.

Notice that the Jews were going to take the Passover on that night.

Joh 18:28  Then they led Jesus from Caiaphas to the Praetorium, and it was early morning. But they themselves did not go into the Praetorium, lest they should be defiled, but that they might eat the Passover.

Exo 12:18  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.