Charles Ellicott Commentary 2 Chronicles 35:15

"And the singers the sons of Asaph were in their place, according to the commandment of David, and Asaph, and Heman, and Jeduthun the king`s seer; and the porters were at every gate: they needed not to depart from their service; for their brethren the Levites prepared for them." — 2 Chronicles 35:15 (ASV)

And the singers the sons of Asaph were in their place. —“At their post” or station (1 Chronicles 23:28). The “sons” of Heman and Jeduthun are omitted for brevity.

According to the commandment ... king’s seer. —Compare to 1 Chronicles 25:1–6.

They might not depart. —Rather, they had no need to depart from their service (i.e., to leave their posts), in order to prepare their own passover and the subsequent meals, for their brethren the Levites had prepared for them, and brought it to them at their several stations.