וְאֵ֣לֶּה הַ֠מְשֹׁרְרִים רָאשֵׁ֨י אָב֧וֹת לַלְוִיִּ֛ם בַּלְּשָׁכֹ֖ת פטירים [פְּטוּרִ֑ים] כִּֽי־יוֹמָ֥ם וָלַ֛יְלָה עֲלֵיהֶ֖ם בַּמְּלָאכָֽה׃
And these are the singers, heads of fathers’ houses of the Levites, who dwelt in the chambers and were free from other service; for they were employed in their work day and night.
Rashi
of those coming in on the Sabbath with those leaving on the Sabbath, for he did not dismiss those divisions who were leaving to go home, but he detained them with those who were arriving. A similarity to [the word פָּטַר] is found in (I Chron. 9:33): “in the chambers, who are exempt (פְּטוּרִים)”; i.e., they were exempt from all work in the forecourt, for day and night it was incumbent upon them to perform the work to guard the forecourts and the chambers; they were therefore exempt from all other work in the forecourt.