John Gill Commentary 2 Chronicles 35

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.

And he set the priests in their charges
In their offices, and in their proper places, to execute them:

and encouraged them to the service of the house of the Lord ;
to attend it with cheerfulness and constancy, and do it according to the will of God, promising his favour and protection.

And said unto the Levites that taught all Israel
Whose business it was to go through the several tribes, and instruct them in the knowledge of God, his word and worship, statutes and ordinances. In an ancient MS. mentioned by Junius, it is read, "who prepared for all Israel the holy things"

which were holy unto the Lord ;
who were consecrated to the name of the Lord, as the Targum, dedicated to his worship and service; or it may signify the holy things they taught or prepared:

these put the holy ark in the house which Solomon the son of David king
of Israel did build ;
which some think was removed from thence by Amon, and an idol put in its room, which is the greater trespass he is said to be guilty of, (2 Chronicles 33:23) others, that it was privately removed by the high priest in idolatrous times, and laid up in some secret place for the preservation of it; but rather the truth is, that it had been removed by the order of Josiah, for the sake of the repairs of the most holy place; and this being done, he orders it to be replaced; and though the Levites might not go into the holy of holies, yet they could carry it to the entrance of the holy place, and the priests from thence to the door of the most holy place, where the high priest could receive it, and fix it in its proper place:

it shall not be a burden upon your shoulders ;
it was not now to be carried from place to place, having a fixed abode in the most holy place, and therefore they were at leisure to attend other service:

serve now the Lord your God, and his people Israel ;
by singing the praises of God, and slaying the passover lambs for the people.

And prepare yourselves To do their work in this service of the sanctuary, the passover; that they be ready to do it, and diligent in it, and perform it according to the law of God:

by the houses of your fathers, after your courses ; such of them whose turn in course it was to officiate:

according to the writing of David king of Israel, and according to the writing of Solomon his son ; who had given in writing directions in what manner their courses should be observed, see (1 Chronicles 23:1–24:31) .

And stand in the holy place
The court of the priests, where their ministrations were:

according to the divisions of the families of your brethren the people ;
of the other tribes, who were according to their families to provide a lamb for the passover:

and after the division of the families of the Levites ;
who were obliged to observe the same ordinance in their respective families, and for whom, as well as for the other families of Israel, they were to slay the lamb.