A SABBATH HIGH DAY

I don't believe the issue is whether or not the first and last days of festivals were sabbaths, the Torah is very clear on that point that those days were sabbaths. The issue is what made a sabbath a "high" day.

A "festival sabbath" was a "lesser" sabbath in that it was not as strict as the weekly sabbath. On festival sabbaths, some work was allowed, that work which was necessary to keep the feast: "And in the first day there shall be an holy convocation, and in the seventh day there shall be an holy convocation to you; no manner of work shall be done in them, save that which every man must do to eat [the feast], that only may be done of you." (Exodus 12:16) So on festivals sabbaths, the work that was necessary to prepare and eat the feast was allowed making these sabbaths less strict than the weekly sabbath.

However, if a festival day, even a festival sabbath fell on the weekly sabbath, then the laws changed. The weekly sabbath commandment overrode the festival commandments and no work could be done at all, not even the work necessary to prepare and eat the feast. That Sabbath was a "high" day in that it overrode all other commandments.

Also, the day before the Passover was never called "the Preparation" (it was and still is called "the eve") simply becuase it was not necessary to make preparations for the Passover the day before, the law allowed all the work necessary to prepare the Passover on the day of Passover. The only day on the Jewish calendar that allowed absolutely no work to be done at all, not even to prepare a feast, was the weekly Sabbath. Consequently, the only day on the Jewish calendar when preparations had to be made the day before was the day before the weekly Sabbath, Fridays.