Albert Barnes Commentary 2 Kings 10:20

"And Jehu said, Sanctify a solemn assembly for Baal. And they proclaimed it." — 2 Kings 10:20 (ASV)

A solemn assembly - Jehu applies to his proposed gathering the sacred name assigned in the Law to the chief festivals of Yahweh (Numbers 29:35; Deuteronomy 16:8).