Albert Barnes Commentary 2 Chronicles 30:23

"And the whole assembly took counsel to keep other seven days; and they kept [other] seven days with gladness." — 2 Chronicles 30:23 (ASV)

To keep other seven days - This was a voluntary addition to the requirements of the Law—the fruit and sign of the abounding zeal that characterized the time. Hezekiah and the princes probably proposed it to the people, and presented them with sacrificial animals.