Chronicles 30:1 Hezekiah sent word to all Israel and Judah and also wrote letters to Ephraim and Manasseh, inviting them to come to the temple of the Lord in Jerusalem and celebrate the Passover to the Lord, the God of Israel. 2 The king and his officials and the whole assembly in Jerusalem decided to celebrate the Passover in the second month. 3 They had not been able to celebrate it at the regular time because not enough priests had consecrated themselves and the people had not assembled in Jerusalem. 4 The plan seemed right both to the king and to the whole assembly. 5 They decided to send a proclamation throughout Israel, from Beersheba to Dan, calling the people to come to Jerusalem and celebrate the Passover to the Lord, the God of Israel. It had not been celebrated in large numbers according to what was written.
6 At the king’s command, couriers went throughout Israel and Judah with letters from the king and from his officials, which read:
“People of Israel, return to the Lord, the God of Abraham, Isaac and Israel, that he may return to you who are left, who have escaped from the hand of the kings of Assyria. 7 Do not be like your parents and your fellow Israelites, who were unfaithful to the Lord, the God of their ancestors, so that he made them an object of horror, as you see. 8 Do not be stiff-necked, as your ancestors were; submit to the Lord. Come to his sanctuary, which he has consecrated forever. Serve the Lord your God, so that his fierce anger will turn away from you. 9 If you return to the Lord, then your fellow Israelites and your children will be shown compassion by their captors and will return to this land, for the Lordyour God is gracious and compassionate. He will not turn his face from you if you return to him.”
10 The couriers went from town to town in Ephraim and Manasseh, as far as Zebulun, but people scorned and ridiculed them. 11 Nevertheless, some from Asher, Manasseh and Zebulun humbled themselves and went to Jerusalem. 12 Also in Judah the hand of God was on the people to give them unity of mind to carry out what the king and his officials had ordered, following the word of the Lord.
13 A very large crowd of people assembled in Jerusalem to celebrate the Festival of Unleavened Bread in the second month. 14 They removed the altars in Jerusalem and cleared away the incense altars and threw them into the Kidron Valley.
15 They slaughtered the Passover lamb on the fourteenth day of the second month. The priests and the Levites were ashamed and consecrated themselves and brought burnt offerings to the temple of the Lord. 16 Then they took up their regular positions as prescribed in the Law of Moses the man of God. The priests splashed against the altar the blood handed to them by the Levites. 17 Since many in the crowd had not consecrated themselves, the Levites had to kill the Passover lambs for all those who were not ceremonially clean and could not consecrate their lambs[a] to the Lord. 18 Although most of the many people who came from Ephraim, Manasseh, Issachar and Zebulun had not purified themselves, yet they ate the Passover, contrary to what was written. But Hezekiah prayed for them, saying, “May the Lord, who is good, pardon everyone 19 who sets their heart on seeking God—the Lord, the God of their ancestors—even if they are not clean according to the rules of the sanctuary.” 20 And the Lord heard Hezekiah and healed the people.
21 The Israelites who were present in Jerusalem celebrated the Festival of Unleavened Bread for seven days with great rejoicing, while the Levites and priests praised the Lord every day with resounding instruments dedicated to the Lord.(b)
22 Hezekiah spoke encouragingly to all the Levites, who showed good understanding of the service of the Lord. For the seven days they ate their assigned portion and offered fellowship offerings and praised[c] the Lord, the God of their ancestors.
23 The whole assembly then agreed to celebrate the festival seven more days; so for another seven days they celebrated joyfully. 24 Hezekiah king of Judah provideda thousand bulls and seven thousand sheep and goats for the assembly, and the officials provided them with a thousand bulls and ten thousand sheep and goats. A great number of priests consecrated themselves. 25 The entire assembly of Judah rejoiced, along with the priests and Levites and all who had assembled from Israel, including the foreigners who had come from Israel and also those who resided in Judah. 26 There was great joy in Jerusalem, for since the days of Solomon son of David king of Israel there had been nothing like this in Jerusalem.27 The priests and the Levites stood to bless the people, and God heard them, for their prayer reached heaven, his holy dwelling place.
a. 2 Chronicles 30:17 Or consecrate themselves
b. 2 Chronicles 30:21 Or priests sang to the Lord every day, accompanied by the Lord’s instruments of praise
c. 2 Chronicles 30:22 Or and confessed their sins to
They didn't have enough priests to handle a major holiday like Passover. Priests were required to slit the throats of the animal and toss it up on the alter. The blood from the animal was drained and that blood was sprinkled on the alter. My description of what the priests had to do may be a little weak but I do see that there weren't enough priests to do the heavy lifting so they had to postpone Passover. But when there were enough priests, cleaned up and consecrated to handle the thousands of sacrifices they would have to number exactly correctly... then the holiday was on and it looks like the Israelites decided to stay an extra week or two.
Enduringword.com has a beautiful piece on this chapter but it's too long. This is from the easy English site.
The king sent letters through the region of the 12 tribes of Israel. The letters even went to Beersheba in the south and to Dan in the north. The letters called all the Israelites to come to Jerusalem for the feast.
In the letters, Hezekiah appealed to the Israelites to turn again to the LORD. Those whom he wrote to had not gone into exile. The reason for the exile was that the people had not been loyal to the LORD their God. Those who had not gone into exile had the chance to repent. If they came back to the LORD, the exiles would come home again. So, Hezekiah invited them to come to the temple in Jerusalem for the Passover feast.
Most of the Israelites laughed and they insulted Hezekiah’s men. But some from Asher, Manasseh and Zebulun did come to Jerusalem. The people in Judah obeyed the king and they came.
Many of the people had not made themselves ready for the Passover. Those people may have touched a dead body. They may have had sex just before the feast. They may not have washed in the proper way. So, the Levites killed the sheep on their behalf. Usually the head of each family would kill the sheep for his family. These people, who were not ready for the feast, did eat of the feast. That was against God’s law. They might have become ill. But Hezekiah prayed for them. He asked the LORD to forgive them. The LORD did forgive them and he cured them.
The people had so enjoyed the feast that they extended it by another 7 days. Hezekiah and his officials provided a lot of bulls and sheep for sacrifices and for food. Many more priests made themselves holy to be able to give all the sacrifices. All the people from Judah and Israel had a great time. Even the foreigners among them were happy.
At the end of the first Passover... the people were horrified. The firstborn of any family that had not sopped the front door frame with the blood of a lamb, died. There were dead everywhere.... even the Pharaoh's son died. This time... not so.... the people were coming back to the Lord. Some were so ignorant, they didn't know the rules and [putting it in today's language] they showed up in shorts or jeans and flip flops. They weren't dressed and ready for worship.... but they were there and that was what was important.... So God accepted their sacrifice [made by a priest cause the priest knew what to do].