Albert Barnes Commentary 2 Chronicles 20:21

"And when he had taken counsel with the people, he appointed them that should sing unto Jehovah, and give praise in holy array, as they went out before the army, and say, Give thanks unto Jehovah; for his lovingkindness [endureth] for ever." — 2 Chronicles 20:21 (ASV)

Praise the beauty of holiness—Some translate this as in the beauty of holiness; that is, in rich attire and ornaments suitable for a holy occasion .