John Gill 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)

And the king commanded all the people
Not at Jerusalem only, but throughout the whole kingdom: saying,

keep the passover unto the Lord your God, as it is written in this book
of the covenant ;
which had been lately found and read, and they had agreed to observe, and in which this ordinance was strictly enjoined, and was a commemoration of their deliverance out of Egypt, and a direction of their faith to the Messiah, the antitype of the passover.