Albert Barnes Commentary 2 Chronicles 30:5

"So they established a decree to make proclamation throughout all Israel, from Beer-sheba even to Dan, that they should come to keep the passover unto Jehovah, the God of Israel, at Jerusalem: for they had not kept it in great numbers in such sort as it is written." — 2 Chronicles 30:5 (ASV)

They had not done it... — Some prefer to interpret this as, "they had not kept it in full numbers, as it was written"—that is, for some time, the Israelites of the northern kingdom had not kept the Passover with the full attendance that the Law required.