Answer
The confusion could be dispelled if we would understand how the NT writers described how the contemporary Jews reckoned the Passover and the days of Unleavened Bread (UB). These are provided in the following verses:
Explanation
Passover and the Feast of UB Interchanged
The New Testament uses Passover and the UB interchangeably. Sometimes it calls the entire 8 days (1 day Passover + 7 days UB) as Passover and sometimes the Feast of UB and sometimes both (as in Mark 14:1 shown later).
“And on the first day of the Feast of Unleavened Bread, the
disciples came to Jesus, saying to Him, Where do you desire we should
prepare for You to eat the Passover?” (Matt 26:17).
Here Matthew clubs together both the Passover on Nissan 14 and the 7 days of UB that begins on Nissan 15, the immediate next day. So, here we see, the 1+7=8 days are called UB!
“And on the first day of the Unleavened Bread, when they killed
the Passover” (Mar 14:12).
Again we see Mark doing the same, that is, the 1 (Passover day) +7 (UB days) = 8 days are called UB.
“And the Feast of Unleavened Bread, BEING CALLED
Passover, drew near” (Luke 22:1).
Here, Luke says that the 1 (Passover day) + 7 (UB days) = 8 days, the entire 8 days are also called Passover. This is what John calls in John 19:14!
"And the day of the Unleavened Bread came, ON WHICH the
Passover must be killed” (Luke 22:7).
So, Luke is very clear: the entire 1 + 7 = 8 days are called either UB days or Passover. If we do not understand this, we could end up confusing the terms. But Mark says that sometimes these 8 days are called both (given below).
The fact is Passover falls on Nissan 14th and the very next day, Nissan 15th, comes the first day of the UB. So the 1 day Passover + 7 days UB are considered to be, for practical purposes, ONE feast. Sometimes, they call it Passover, sometimes the UB and sometimes, both as in Mark 14:1:
“And it was the Passover, AND (the Feast) of Unleavened
(Bread) after two days”.
So, we see:
Nissan 14 – Passover, a working day;
Nissan 15 – first day of UB; an annual Sabbath (a high day); no work;
Nissan 16 – second day of UB; a working day;
Nissan 17 – third day of UB; a working day;
Nissan 18 – fourth day of UB; a working day;
Nissan 19 – fifth day of UB; a working day;
Nissan 20 – sixth day of UB; a working day;
Nissan 21 – seventh day of UB; an annual Sabbath (a high day); no work.
[Since UB covers 7 days, the Saturday Sabbath can fall on any day and that day will be a Sabbath rest “according to the commandment” (Luke 23:56) and no work should be done. The next Sunday morning is the Wave Sheaf offering, a working day]
Preparation Day
Since Passover (Nissan 14) was a working day that immediately preceded the UB and the first day of UB (Nissan 15) was an annual Sabbath on which no work was to be done, Passover day came to be regarded as “the day of Preparation”.
Jesus, as a devout Jew who esteemed and obeyed God’s laws and commandments (but not all the Jewish traditions), observed all the feast days.
Nissan 14 (Tuesday evening) – Jesus ate the Passover at the evening;
Nissan 14 (Wednesday daytime) – Jesus, as “our Passover Lamb” (1 Cor 5:7) was sacrificed during the day. It is a working day. At evening Jesus was buried in a hurry. Preparation day of UB;
Nissan 15 (Thursday daytime) – first day of UB, an annual Sabbath. “The next day after the day of preparation” the chief priests and the Pharisees came to Pilate. It was well within “a Sabbath day’s journey” (Acts 1:12). So they thought they didn’t break Sabbath according to their tradition.
But they went and secured the sepulcher on the high Sabbath. No wonder, Jesus criticized them for keeping the Sabbath according to their convenience (Luke 14:5).
All these happened during the day time.
Conclusion
Since we have seen clearly that 1 Passover day + 7 days of UB = 8 days is interchangeably called either UB feast or Passover or both, we can understand the following verses thus:
John 19:31 –
“Then, since it was Preparation (Nissan 14 Passover day ending), that
the bodies not remain on the cross on the Sabbath (Nissan 15 UB), for
great was the day of that Sabbath, the Jews asked Pilate that their
legs might be broken and they be taken away”.
Luke 22:7 –
“And the day of the Unleavened Bread came (Nissan 14 evening), on
which the Passover must be killed (Nissan 14 evening)”.