C. F. Keil and Franz Delitzsch on 2 Chronicles 30:21

2Ch 30:21

And the Israelites that were present at Jerusalem kept the feast of unleavened bread seven days with great gladness; and the Levites and priests praised the Lord day by day, singing to the Lord ליהוה עז בּכלי, “with instruments of power to the Lord,” i.e., with which they ascribed power to the Lord; or, to express it more clearly, which they played to the praise of the power of the Lord. The stringed instruments played by the Levites, and the trumpets blown by the priests, to accompany the psalm-singing, are meant. The singing of praise in connection with the sacrificial service took place on the seventh day of the feast.