Now that the Temple is cleansed and sanctified, it's time for a major celebration.
First, a little background. Between 740 and 722 BCE, the Assyrian Empire attacked the Northern Kingdom of Israel. Most of the people were deported until finally the Assyrian king took control of Samaria, the capital of Israel.
It seems that this all happened during Hezekiah's rule of Judah. This may be why he decides to offer an olive branch to anyone still left in the Northern Kingdom. He invites them to celebrate Passover with them.
He sends out messengers all over Judah and Israel to tell the people to come on down to Jerusalem.
King Hezekiah asks the northerners to return to the fold. He assures him that if they return to God, God will accept them back.
Even though things are terrible in Israel, the messengers are pretty much ridiculed wherever they go.
There are a couple stragglers who are humble enough to admit that they've gone astray, but everyone else is going to be in big trouble.
Naturally, all of Judah comes and the celebration is epic.
The people get to work making sure that everything is perfect. Priests slaughter the Passover lamb and sanctify themselves and the people.
Mistakes are made (some people eat Passover food without going through the appropriate rituals to cleanse themselves), but God is so pleased with how hard everyone's trying that he's cool with looking the other way this time.
Everything else happens exactly according to the laws of Moses.
The residents of Israel and Judah hadn't celebrated together like this since the time of Solomon, so this was a very big deal.