2 Kings 23:21

And the king commanded all the people, saying, Keep the passover unto the LORD your God, as it is written in the book of this covenant.

The king commanded ... Keep the Passover ... It was observed with great solemnity, and was attended not only by his own subjects, but by the remnant people from Israel (see the notes at 2 Chronicles 35:1). Many of the Israelites who were at Jerusalem might have heard of, if they did not hear, the law read by Josiah. It is probable that they might even have gotten a copy of the law, stimulated as they were to the better observance of Yahweh's worship by the unusual sad solemn transactions at Jerusalem.