The Passover is a ceremony recalling the deliverance of the nation of Israel from Egyptian slavery. It was instituted by God in commandments to Moses. The original Passover was held in Egypt. The details are given in Exodus 12.
The command to keep the Passover was repeated by Moses in Deuteronomy. It was one of the three mandatory feasts that Israel were expected to keep:
1 Observe the month of Abib, and keep the Passover to the LORD your God; for in the month of Abib the LORD your God brought you out of Egypt by night.
An exceptional Passover was held in the time of Hezekiah:
15 Then they killed the Passover on the fourteenth day of the second month. The priests and the Levites were ashamed, and sanctified themselves, and brought burnt offerings into the LORD’s house.
26 So there was great joy in Jerusalem; for since the time of Solomon the son of David king of Israel there was nothing like this in Jerusalem.
And in the reign of king Josiah:
1 Josiah kept a Passover to the LORD in Jerusalem. They killed the Passover on the fourteenth day of the first month.
18 There was no Passover like that kept in Israel from the days of Samuel the prophet, nor did any of the kings of Israel keep such a Passover as Josiah kept, with the priests, the Levites, and all Judah and Israel who were present, and the inhabitants of Jerusalem.
The feast of Passover pre-figured the ultimate salvation that would be achieved through Jesus, whose death is described as a sacrifice. The significance of unleavened bread eaten with the Passover lamb is also explained by the apostle Paul:
7 Purge out the old yeast, that you may be a new lump, even as you are unleavened. For indeed Christ, our Passover, has been sacrificed in our place. 8 Therefore let’s keep the feast, not with old yeast, neither with the yeast of malice and wickedness, but with the unleavened bread of sincerity and truth.