Hezekiah invited all Israel, even Ephraim and Manasseh, to Jerusalem to keep which feast?

Answer: Passover

Hezekiah sent letters throughout Israel and Judah, from Beersheba to Dan, calling the people to keep the Passover in Jerusalem.

“Then Hezekiah sent word throughout all Israel and Judah, and he also wrote letters to Ephraim and Manasseh inviting them to come to the house of the LORD in Jerusalem to keep the Passover of the LORD, the God of Israel.”— 2 Chronicles 30:1 (BSB)