Adam Clarke on 2 Chronicles 30:2

In the second month - In Ijar, as they could not celebrate it in Nisan, the fourteenth of which month was the proper time. But as they could not complete the purgation of the temple, till the sixteenth of that month, therefore they were obliged to hold it now, or else adjourn it till the next year, which would have been fatal to that spirit of reformation which had now taken place. The law itself had given permission to those who were at a distance, and could not attend to the fourteenth of the first month, and to those who were accidentally defiled, and ought not to attend, to celebrate the passover on the fourteenth of the second month; see Num 9:10, Num 9:11. Hezekiah therefore, and his counsellors, thought that they might extend that to the people at large, because of the delay necessarily occasioned by the cleansing of the temple, which was granted to individuals in such cases as the above, and the result showed that they had not mistaken the mind of the Lord upon the subject.