That particular Sabbath is regarded as a "High" Sabbath because it falls during the feast of unleavened bread. Remember Jesus was arrested on the night of the Passover and the 7 Day feast of unleaved bread starts the day after Passover. Passover and the Feast of Unleavened bread are 2 of the 7 required feasts outlined in Leviticus Chapter 23.
To clarify Stevens response somewhat...the first day and the last day of the feast of Unleavened bread are sabbaths in and of themselves, but are not the same as the weekly sabbaths unless they happen to fall on the weekly sabbath.
Another word is High Holy day. It was a high sabbath because it wasn't the regular weekly sabbath.