"That Sabbath was a high day", not "that day was a high Sabbath"
This is because the holy convocation happened to fall on the weekly Sabbath. Note that on the weekly Sabbath and some of the other holy convocations described in Leviticus 23, they were not to do work of any kind, however for the passover they were not to do any servile work. This meant they were allowed to prepare food, unlike the weekly Sabbath where food had to be prepared the day before on what is known as the day of preparation, what we call Friday. Since the food can be prepared on the Passover sabbath, there is no day of preparation preceding it, however the Gospel accounts say the day on which Christ was crucified was the day of preparation which means it was the day before the weekly Sabbath. Hence the Passover sabbath that year fell on the weekly Sabbath, making that Sabbath a "high day"
