en_tn/2ch/30/21.md

kept the Festival of Unleavened Bread

Here "kept the festival" is a idiom meaning to participate in the festival or celebrate the festival. AT: "celebrate the festival" (See: rc://en/ta/man/translate/figs-idiom)

singing with loud instruments to Yahweh

"singing and playing loud musical instruments to Yahweh"

who understood the service of Yahweh

This was the reason that he spoke encouragingly to them. AT: "because they understood the service of Yahweh" or "because they served Yahweh skillfully" (See: rc://en/ta/man/translate/figs-distinguish)

So they ate throughout the festival

The word "they" refers to the people of Israel.

making confession to Yahweh

Possible meanings are 1) "confession" here refers to saying that God is great. AT: "praising Yahweh" or 2) "confession" here refers to admitting their sins. AT: "confessing their sins to Yahweh"