דברי הימים ב, פרק ל״ה, פסוק א׳

The renewal of the Passover sacrifice under King Josiah marks a powerful return to the exact observance of the Torah's commandments following a long era of neglect. Josiah was driven to observe the holiday precisely because the wicked kings of Judah who preceded him had entirely failed to keep the festival as required by law [רש״י].

Every physical action, including the slaughtering of the sacrifices, was carried out with strict attention to the original commands [ביאור שטיינזלץ]. The timing of the event is also highly significant, as the observance took place on its exact, original date on the fourteenth day of the first month. This stands in sharp contrast to the earlier reign of King Hezekiah, who was forced by circumstance to delay the observance until the second month. Josiah, however, succeeded in fulfilling the obligation in the first month, restoring the holiday exactly as the original law dictates [מצודת דוד].