Albert Barnes Commentary 2 Kings 23:21

"And the king commanded all the people, saying, Keep the passover unto Jehovah your God, as it is written in this book of the covenant." — 2 Kings 23:21 (ASV)

See the note on 2 Kings 23:4. With this verse, the author returns to the narrative of what was done in Josiah’s eighteenth year.

The need for the command, as it was written in the book of this covenant, was not because Josiah had not yet held a Passover. Rather, it was because reading the book had revealed to him discrepancies between the current practice and the letter of the Law. These differences resulted from negligence or from tradition being allowed, in various points, to override the Law.