John Gill Commentary 2 Chronicles 29:3

"He in the first year of his reign, in the first month, opened the doors of the house of Jehovah, and repaired them." — 2 Chronicles 29:3 (ASV)

He in the first year of his reign, in the first month
And, as appears from (2 Chronicles 29:17) , on the first day of the month:

opened the doors of the house of the Lord ;
which his father had shut, (2 Chronicles 28:24)

and repaired them ;
or strengthened them, the hinges, and other parts of them, being loosened and weakened; and ornamented them by overlaying them with gold, the plates of which very probably his father had taken off; for, certain it is, Hezekiah overlaid them, and very probably at this time, see (2 Kings 18:16) .