The cleansing and reconsecration of the temple and priesthood wasn't finished until the 16th day of the first month of the Hebrew calendar (2 Chronicles 29:17 2 Chronicles 29:17Now they began on the first day of the first month to sanctify, and on the eighth day of the month came they to the porch of the LORD: so they sanctified the house of the LORD in eight days; and in the sixteenth day of the first month they made an end.
American King James Version×). This was during the Days of Unleavened Bread, two days after the date for the Passover, which was to be kept on the 14th day of the first month (see Leviticus 23:4-8 Leviticus 23:4-8  These are the feasts of the LORD, even holy convocations, which you shall proclaim in their seasons.  In the fourteenth day of the first month at even is the LORD's passover.  And on the fifteenth day of the same month is the feast of unleavened bread to the LORD: seven days you must eat unleavened bread.  In the first day you shall have an holy convocation: you shall do no servile work therein.  But you shall offer an offering made by fire to the LORD seven days: in the seventh day is an holy convocation: you shall do no servile work therein.
American King James Version×). So Hezekiah and the people had missed the Passover and the start of the Feast of Unleavened Bread.
But in Numbers 9:9-12 Numbers 9:9-12  And the LORD spoke to Moses, saying,
 Speak to the children of Israel, saying, If any man of you or of your posterity shall be unclean by reason of a dead body, or be in a journey afar off, yet he shall keep the passover to the LORD.
 The fourteenth day of the second month at even they shall keep it, and eat it with unleavened bread and bitter herbs.
 They shall leave none of it to the morning, nor break any bone of it: according to all the ordinances of the passover they shall keep it.
American King James Version×God had made provision for postponing the Passover to the 14th day of the second month for those who were unable to observe the regular Passover because they were far away on a journey or ritually unclean. Hezekiah "and his leaders" applied this rule to the priests, since they were not yet all consecrated—and by extension to the people, since they needed the priesthood (2 Chronicles 29:2-3 2 Chronicles 29:2-3  And he did that which was right in the sight of the LORD, according to all that David his father had done.  He in the first year of his reign, in the first month, opened the doors of the house of the LORD, and repaired them.
American King James Version×). Thus, the festival would be observed this year in the second month. It should be noted that among Hezekiah's leaders was probably Isaiah the prophet, who was no doubt receiving instructions from God during this time.
Hezekiah even extends an invitation to those in the northern kingdom to join the people of Judah in keeping the Passover and participating in this religious revival. As in the previous chapter, we again see the phrase "all Israel," now with the addition of "from Beersheba to Dan" (verse 5), which was a common way of denoting the whole land of Israel prior to the divided monarchy—Beersheba being located in the far south and Dan being the extreme north. Though most of Israel had been taken captive in 733-732 B.C., Israel's final deportation had not yet taken place and there was still a sizeable remnant population left at this time as part of the kingdom ruled by Hoshea, himself a vassal of Assyria.
Notice that the runners from Judah go only as far north as Zebulun (verse 10). This was evidently the northernmost territory of the remnant state of Israel at this time, the inhabitants of Naphtali to the north of Zebulun having already been deported by Tiglath-Pileser III (compare 2 Kings 15:29 2 Kings 15:29In the days of Pekah king of Israel came Tiglathpileser king of Assyria, and took Ijon, and Abelbethmaachah, and Janoah, and Kedesh, and Hazor, and Gilead, and Galilee, all the land of Naphtali, and carried them captive to Assyria.
American King James Version×). Indeed, this is actually more proof that Hezekiah's Passover should be dated to the time before Israel's final deportation in 722 B.C. rather than after it.
Sadly, despite the invasion and captivity the Israelites have already suffered at this point, most of those who are left in the land do not take warning and repent. The people of the northern kingdom are so far from God that very few are interested in observing God's Passover. Indeed, they mock the messengers. But a few do respond (2 Chronicles 30:10-11 2 Chronicles 30:10-11  So the posts passed from city to city through the country of Ephraim and Manasseh even to Zebulun: but they laughed them to scorn, and mocked them.
 Nevertheless divers of Asher and Manasseh and of Zebulun humbled themselves, and came to Jerusalem.
American King James Version×).
Then came the actual observance of the Passover. While Hezekiah and the priests were very careful to do all that was required in the law of God (verse 16), some in the assembly, particularly those who did come down from the northern tribes, were not appropriately prepared for it. While the Passover was normally slain by the heads of household, and the priests only sprinkled the blood on the altar, in this case the Levites killed the Passover for those who were not clean (verse 17).
Even then, eating the Passover was technically not permitted (compare Numbers 9:6-7 Numbers 9:6-7  And there were certain men, who were defiled by the dead body of a man, that they could not keep the passover on that day: and they came before Moses and before Aaron on that day:
 And those men said to him, We are defiled by the dead body of a man: why are we kept back, that we may not offer an offering of the LORD in his appointed season among the children of Israel?
American King James Version×), but because of the special circumstances, and the fact that this was already the second Passover, Hezekiah prayed that God would look on the hearts of the people and forgive this transgression. And God did, "healing" the people—that is, their relationship with Him (2 Chronicles 30:18-20 2 Chronicles 30:18-20  For a multitude of the people, even many of Ephraim, and Manasseh, Issachar, and Zebulun, had not cleansed themselves, yet did they eat the passover otherwise than it was written. But Hezekiah prayed for them, saying, The good LORD pardon every one  That prepares his heart to seek God, the LORD God of his fathers, though he be not cleansed according to the purification of the sanctuary.  And the LORD listened to Hezekiah, and healed the people.
American King James Version×). The people also went on to keep the entire Feast of Unleavened Bread in the second month—and, moved by the religious fervor and revival, went on to keep yet another seven days of feasting and rejoicing.
Notice the emphasis on the joyous and zealous attitudes of the people: "who prepares his heart to seek God" (verse 19); "singleness of heart" (verse 12); "with great gladness" (verse 21); "with gladness" (verse 23); "great joy" (verse 26).
It is somewhat revealing to note that such major observances of the festivals at Jerusalem, while mandated by God, were not common during the monarchy. We are told that such a celebration had not taken place since the days of Solomon (verse 26). And a later Passover by Josiah is described as being unlike anything since the days of Samuel (2 Chronicles 35:18 2 Chronicles 35:18And there was no passover like to that kept in Israel from the days of Samuel the prophet; neither did all the kings of Israel keep such a passover as Josiah kept, and the priests, and the Levites, and all Judah and Israel that were present, and the inhabitants of Jerusalem.
American King James Version×).
As the people returned home from two incredible weeks of spiritual recommitment and rejuvenation, they proceeded to destroy pagan shrines and images "until they had utterly destroyed them all"—even throughout the territories of Ephraim and Manasseh in the northern kingdom (2 Chronicles 31:1 2 Chronicles 31:1Now when all this was finished, all Israel that were present went out to the cities of Judah, and broke the images in pieces, and cut down the groves, and threw down the high places and the altars out of all Judah and Benjamin, in Ephraim also and Manasseh, until they had utterly destroyed them all. Then all the children of Israel returned, every man to his possession, into their own cities.
American King James Version×). This was a remarkably bold undertaking fraught with danger. For since only a few from the northern kingdom had responded positively to Hezekiah's invitation, this surely would not have gone over well there at all.
Hezekiah is praised for at last removing the high places, or pagan worship sites, from the land (2 Kings 18:4 2 Kings 18:4He removed the high places, and broke the images, and cut down the groves, and broke in pieces the brazen serpent that Moses had made: for to those days the children of Israel did burn incense to it: and he called it Nehushtan.
American King James Version×)—which, as was often lamented, even numerous righteous kings before him had failed to do. And, surprisingly, we also see him being praised for destroying something that God had told Moses to make. The bronze serpent had become an idolatrous image that people worshiped. So, as it had outlived its intended purpose and was now inextricably linked with sinful idolatry, a decision was made to destroy it. Indeed, it is possible that God Himself gave the order through Isaiah or through Hezekiah consulting the high priest with the Urim and Thummim.