C. F. Keil and Franz Delitzsch on 2 Chronicles 30:1

2Ch 30:1 The celebration of the passover. - 2Ch 30:1-12. The preparations for this celebration. - 2Ch 30:1. Hezekiah invited all Israel and Judah to it; “and he also wrote letters to Ephraim and Manasseh,” the two chief tribes of the northern kingdom, which here, as is manifest from 2Ch 30:5, 2Ch 30:10, are named instar omnium. But the whole sentence serves only to elucidate כּל־שׂראל על ישׁלה. To all Israel (of the ten tribes) he sent the invitation, and this he did by letters. The verse contains a general statement as to the matter, which is further described in what follows.