Your lamb shall be without blemish, a male a year old. You may take it from the sheep or from the goats, and you shall keep it until the fourteenth day of this month, when the whole assembly of the congregation of Israel shall kill their lambs at twilight.Leviticus 23 and Numbers 28 likewise instruct the Hebrews concerning Passover and the Feast of Unleavened Bread.
“Then they shall take some of the blood and put it on the two doorposts and the lintel of the houses in which they eat it. They shall eat the flesh that night, roasted on the fire; with unleavened bread and bitter herbs they shall eat it. (Exo 12:5-8)
And you shall observe the Feast of Unleavened Bread, for on this very day I brought your hosts out of the land of Egypt. Therefore you shall observe this day, throughout your generations, as a statute forever. In the first month, from the fourteenth day of the month at evening, you shall eat unleavened bread until the twenty-first day of the month at evening (Exo 12:17-18)
Passover and the Feast of Unleavened Bread were intimately connected. Passover refers to the angel passing over the Israelite dwellings. Yet the lamb was to be eaten with unleavened bread from the first day. God gave instructions about the passover lamb and the unleavened bread but the festival was a single festival. As such both the term "Passover" and the term "Feast of Unleavened Bread" were somewhat synonymous.
The synoptic gospels make it clear that the Passover was celebrated by Jesus and his disciples on Thursday evening which would have been Nisan 14. He was crucified on the Friday which was still Nisan 14 (sunset to sunset reckoning). The only difficulties with this position concern John's chronology and the term 3 days and 3 nights. I have argued that the latter phrase is an idiom. Were we to take this as a literal time frame of 72 hours we are left with the problem of Jesus rising on the third day.
Concerning John's chronology he writes
Then they brought Jesus from Caiaphas to the governor’s residence. Now it was early, and they did not enter into the governor’s residence so that they would not be defiled, but could eat the Passover. (Joh 18:28)Previously I have suggested that Day of Preparation is 6th day of the week, the day that the Jews prepare for the Sabbath. The phraseology sounds like it is a day for preparing for the Passover. However Passover is a synonymous with the Feast of Unleavened Bread. Note how Luke says,
Now it was the day of preparation of the Passover (Joh 19:14)
Now the Feast of Unleavened Bread, which is called Passover, was drawing near. (Luk 22:1)It appears that John is using Passover to refer to the week long feast. Day of Preparation is our Friday. To paraphrase, John is saying that it was about the 6th hour on Friday of Passover week.