2 For the feast is not on account of the days; but for the Lord's sake, who then suffered for us, we celebrate it, for 'our Passover Christ, is sacrificed.' Even as Moses, when teaching Israel not to consider the feast as pertaining to the days, but to the Lord, said, 'It is the Lord's Passover.' To the Jews, when they thought they were keeping the Passover, because they persecuted the Lord, the feast was useless; since it no longer bore the name of the Lord, even according to their own testimony. It was not the Passover of the Lord, but that of the Jews. The Passover was named after the Jews, my brethren, because they denied the Lord of the Passover. On this account, the Lord, turning away His face from such a doctrine of theirs, says, 'Your new moons and your sabbaths My soul hates.'
Source: Letters (New Advent)