Charles Ellicott Commentary 2 Kings 10:20

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

Proclaim a solemn assembly. —Rather Sanctify a solemn meeting (Isaiah 1:13). Every person who wished to attend would have to “sanctify,” or purify, himself in due form.

They proclaimed —that is gave notice of the festival by criers through all Israel (2 Kings 10:21).