Note 2 at Joh 18:28: The first day of Passover was a holy convocation, or a special Sabbath (see note 1 at Joh 2:13). It may be that these Jews considered participation in a trial as work, and that would defile them and make them unable to eat the Passover. If this were so, it was certainly hypocritical on their part, since they would have already been defiled by conducting the religious trial of Jesus just a few hours earlier.

It is also possible that they were observing some oral tradition about being defiled by just keeping company with a Gentile. Peter made mention of this tradition when he went to Cornelius' house. However, there are no Old Testament scriptures that expressly back up that tradition.

Whatever their reasoning, this is a classic example of straining at a gnat and swallowing a camel (Mt 23:24). They were so careful to avoid ceremonial uncleanness, but they were in the very process of the greatest defilement of all time by condemning the sinless Son of God.