This was when the Israelites brought the ark of the covenant into the temple. From II Chronicles 5 --
12 All the Levites who were musicians—Asaph, Heman, Jeduthun and their sons and relatives—stood on the east side of the altar, dressed in fine linen and playing cymbals, harps and lyres. They were accompanied by 120 priests sounding trumpets. 13 The trumpeters and singers joined in unison, as with one voice, to give praise and thanks to the LORD. Accompanied by trumpets, cymbals and other instruments, they raised their voices in praise to the LORD and sang:
"He is good;
his love endures forever."
Then the temple of the LORD was filled with a cloud, 14 and the priests could not perform their service because of the cloud, for the glory of the LORD filled the temple of God.
Can you imagine standing there ready to offer the sacrifices, but being mesmerized by God's overwhelming glory? What a wondrous experience!