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]])
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"