Charles Ellicott Commentary 1 Kings 21:9

"And she wrote in the letters, saying, Proclaim a fast, and set Naboth on high among the people:" — 1 Kings 21:9 (ASV)

Proclaim a fast. —This might have been intended only to cover all that was to be so foully done with a cloak of religious observance. Or, perhaps more probably, it was meant to imply that some secret sin had been committed, which would bring down vengeance on the whole city, and so to prepare for the false accusation. There is a similar ambiguity regarding the explanation of the command, set Naboth on high, as either an exaltation of pretended honour, or the lifting up his head (Genesis 40:20) for accusation. It may be noted that the whole scheme implies a return of the people to at least the outward observance of the Law of the Lord.