Lesley, Your question is excellent and you are correct.
"My question is based on John 19:31 which says that on the day Jesus died it was the day of Preparation, and the next day was to be a special Sabbath. Was this special Sabbath (on which no work could be done) the first day of the Feast of Unleavened Bread?"
Yes, on both counts. Passover is the 14th of Nisan, the 15th of Nisan starts the Feast of Unleavened bread, but since they are back to back and the feast of Unleavened bread lasts 7 days, they all get lumped together as Passover.
This is very significant, because this is exactly what the reason for so much confusion and misinformation about the crucifixion - most Christians today don't understand Jewish culture, or Jewish calendars, or Feast days.
Another very important thing that most Christians don't realize is that in the Hebrew language, Shabbat is the word for Sabbath, but all the 7 feasts and the weekly sabbath are Shabbat. Passover is a feast day, but it's not a High Sabbath day - in which all work is forbidden. Slaughtering a lamb, cleaning it and roasting it is a lot of work, and Passover is the preparation day for Feast of Unleavened Bread - in which no work could be done. Church leaders incorrectly assumed that this was Friday - preparation for the weekly sabbath.
Regarding your second question, Lesley, yes, there is evidence, it is clearly in the gospel account itself, and also in the Old Testament Torah, specifically Leviticus, where the feasts are given along with the specific commands about how they are to be kept.
https://www.gotquestions.org/high-Sabbath.html "So Jesus was crucified on the day before a high Sabbath. Because the Jews did not want Jerusalem desecrated on such a holy day as a high Sabbath, they petitioned the governor that the bodies of Jesus and those crucified with Him be taken down before evening (see Deuteronomy 21:22–23).
Scripture explicitly says that there were two Sabbaths in Christ's final week. The High Sabbath being Nisan 15 [Wed sundown -Thursday day] ** Remember Jewish days go sundown to sundown - Evening and Day. Jews used a Lunar calendar, and the cool thing is that the new moon is the start of every month, and because Passover is always the 14th of Nisan, it means Passover is always on a full moon. The Pharisees asked Christ for a sign that he really was the Messiah, and he only gave one sign and this was the sign of Jonah, which Christ quoted in Matthew.
"Just as Jonah was 3 days and 3 nights in the belly of the great fish, so the son of man will be 3 days and 3 nights in the heart of the earth."
Scripture says that the women bought rested on the sabbath and bought spices after the sabbath.
This is impossible to buy spices after dark on Saturday night - after the weekly Sabbath. They rested on High Sabbath - Thursday, bought spices on Friday, after the High Sabbath, and before the weekly Sabbath.
In the 1st century, the only light besides the sun and moon were candles and torches. Buying and selling was forbidden on Sabbaths. So the idea of buying spices after sunset on Saturday night is patently absurd. No one would go set up their goods to sell in the dark, after the Sabbath had ended.
So they rested on the High Sabbath-feast of Unleavened bread- Thursday. Bought spices on Friday, and then rested on the weekly sabbath.
This is also why they had to get the bodies down from the cross before the high Sabbath - Feast of unleavened Bread. Wednesday at sunset to Saturday at Sunset is 3 days and 3 nights - just like Jesus himself said.
The other interesting thing is that we have 3 separate accounts of people being dead for at least 3 full days and nights, and raising again. Lazarus, Jesus, and the 2 witnesses in Revelation. This is because according to Jewish world view, and burial customs - the spirit lingers near the body for 3 days - so they would not be fully dead, but in a coma. After 3 days, the soul or spirit leaves so there they would be truly dead, and no one would say they were in a coma, and thus- no chance of resurrecting. This is what makes the resurrection so amazing.
Either we can trust the words Christ said, and the only sign he gave that he was the Messiah, or we can't. It's impossible to get 3 days and 3 nights from 1 day and parts of 2 days. It also requires that we totally disregard the clear words of scripture and even the words of Christ himself and a Friday crucifixion cannot answer the problem of the spices and 3 days and 3 nights.
An excellent summary and solid explanation with charts is here on blue letter Bible. https://www.blueletterbible.org/faq/crux.cfm