John Gill Commentary 2 Chronicles 35:17

"And the children of Israel that were present kept the passover at that time, and the feast of unleavened bread seven days." — 2 Chronicles 35:17 (ASV)

And the children of Israel that were present kept the passover
at that time
In its proper time, on the fourteenth day of Nisan:

and the feast of unleavened bread seven days ;
the seven days following the passover, as the Lord by Moses appointed.