John Gill Commentary 2 Chronicles 35:1

"And Josiah kept a passover unto Jehovah in Jerusalem: and they killed the passover on the fourteenth [day] of the first month." — 2 Chronicles 35:1 (ASV)

Moreover, Josiah kept a passover unto the Lord in Jerusalem ,
&c.] Where only it was to be kept:

and they killed the passover on the fourteenth day of the first month ;
the month Nisan, as the Targum, which was the exact time of killing the passover lamb, according to the law of Moses, (Exodus 12:6) , in the Vulgate Latin version of the Apocrypha in: ``And Josias held the feast of the passover in Jerusalem unto his Lord, and offered the passover the fourteenth day of the first month;''

it is called the fourteenth moon of the first month; a phrase often used in ecclesiastical writers, when speaking of the time of the passover; and so we now call one of the days of the week "dies lunae", Monday.