For more information on the "Day of Preparation for the Passover," see post #158.
The Passover lamb was sacrificed and roasted in the evening of the 14th, and then eaten the night of the 14th.
The following Passover day of the 14th was the Day of Preparation for the Feast of Unleavened Bread, and the day Jesus was crucified.
The clear evidence for the above dates is the testimony of all four gospels that
1) Jesus ate the Passover meal before he died, and
2) that he died on the day before the Sabbath, and
3) that he rose on the first day of the week, Sunday.
There is only way these reported facts add up:
1) he died on the 14th, Passover (day) Friday.
2) he was in the grave on the 15th (Saturday Sabbath of Unleavened Bread--no Wednesday Passover or Sabbath), and
3) he rose on the 16th (first day of the week, Sunday).
All four gospels give this account locating Jesus' death after the Passover meal.
Passover in the time of Jesus could mean three things:
1) Passover - Nisan 14, one-day feast
2) Unleavened Bread - Nisan 15-21, seven-day feast
3) both feasts together - Nisan 14-21, eight days of feasts
And none of these are used the same in the different gospels.
Then Unleavened Bread in the NT could mean two things:
1) Unleavened Bread - Nisan 15-21, seven-day feast, or
2) both feasts - Nisan 14-21, eight days of feasts
By the time of the NT, Unleavened Bread was often called Passover, but that is not the name it was given when it was legislated in Lev 23:4-8.
It seems the legislated names were correctly used for 800-900 years before there was abbreviation of them into just Passover to refer to any of the three meanings given above, in second response.
But by the time of the NT, the abbreviations were virtually interchangeable, and have to be sorted out according to their origins in the OT, where the legislation of them
in Leviticus gives the correct nomenclature and understanding of them, and then all four gospels give the actual practice of them in Jesus' lifetime.
There is interesting information on Jesus' connection to Passover in posts #178 and #179, brought to mind by Onlooker's posts.