23:31a Whenever burnt offerings were presented to the LORD on the Sabbaths,

They also were to do that when the priests offered burnt offerings to Yahweh. That happened on Sabbath days,
-or-
They also had to do it every time when burnt offerings were brought to Yahweh. Those offerings were brought every Sabbath day,

23:31b New Moons, and appointed feasts,

at New Moon festivals and on other special days. (EEB)
-or-
when a new moon appeared and at other special days.

23:31c they were to serve regularly before the LORD in the numbers

The correct number of Levites had to be there to serve Yahweh at all times,
-or-
They were told how many of them should be there (Translation for Translators)

23:31d prescribed for them.

as the rules commanded. (EEB)
-or-
and what they should do each time. (Translation for Translators)

23:31a–d (combined)

They also had to sing when the priests gave sacrifices by fire to the Lord on special days. That included the Sabbaths and the New Moon sacrifices. (Easy English Bible) The correct number of Levites had to do everything in the proper way. They were to always work in a regular way and at the correct times in front of the Lord.

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