Josiah had them celebrate the Passover Festival

21 Then the king commanded all the people to celebrate the Passover Festival to honor Yahweh their God, which was written in the law of Moses that they should do every year. 22 During all the years that leaders ruled Israel and during all the years that kings had ruled Israel and Judah, they had not celebrated that festival. 23 But now, after Josiah had been ruling for almost 18 years, to honor Yahweh they celebrated the Passover Festival in Jerusalem.