Charles H. Spurgeon on Psalms 81:3

Blow up the trumpet in the new moon. Announce the sacred month, the beginning of months, when the Lord brought his people out of the house of bondage. Clear and shrill let the summons be which calls all Israel to adore the Redeeming Lord. In the time appointed, on our solemn feast day. Obedience is to direct our worship, not whim and sentiment: God's appointment gives a solemnity to rites and times which no ceremonial pomp or hierarchical ordinance could confer. The Jews not only observed the ordained month, but that part of the month which had been divinely set apart. The Lord's people in the olden time welcomed the times appointed for worship; let us feel the same exultation, and never speak of the Sabbath as though it could be other than "a delight" and "honourable." Those who plead this passage will keep such feasts as the Lord appoints, but not those which Rome or Canterbury may ordain.