The people had not been able to celebrate the Passover Festival at the proper time in the first month, because not enough priests were ritually clean and not many people had assembled in Jerusalem. So King Hezekiah, his officials, and the people of Jerusalem agreed to celebrate it in the second month, and the king sent word to all the people of Israel and Judah. He took special care to send letters to the tribes of Ephraim and Manasseh, inviting them to come to the Temple in Jerusalem to celebrate the Passover in honor of the Lord, the God of Israel. 1
References for 2 Chronicles 30:1
30.2, 3 Nu 9.9-11.