2 Chronicles 30:3

For they could not keep it at the time
In the month Nisan, as the Targum adds, on the fourteenth day of the month, as the law enjoined, because the cleansing of the temple was not finished until the sixteenth day, see ( 2 Chronicles 29:17 ) and, besides this, two other reasons follow:

because the priests had not sanctified themselves sufficiently;
that is, a sufficient number of them were not sanctified, to slay all the passover lambs the people that came to the feast would want:

neither had the people gathered themselves together to Jerusalem;
they had no notice of it, nor summons for it; and it was required that, at such a time, all the males in the land should appear at Jerusalem; but this custom having been long disused, it required time to acquaint them of the revival of it.