Priestly service. Or, if the verse is speaking of a Levite, singing (Arakhin 11a).
|whose turn it is|
Literally, 'who stand.' The priests were divided into different shifts, with each having a different day to serve (Yad, K'ley HaMikdash 4:3).
|On the festivals...|
(Yad, K'ley HaMikdash 4:4; Sukkah 55b).
|The only exception...|
That is, sacrifices that are not brought especially for the festival (Yad, K'ley HaMikdash 4:5). However, some interpret this verse, 'The Levitical priest who lives in any of your Israelite settlements can come to the place that God shall choose whenever he desires. (18:7) He can then serve before God his Lord, just the same as any of his fellow Levitical priests who stand and serve before God. (18:8) He shall then receive the same portion to eat as they do, with the exception of those portions that have been sold as an ancestral right' (Ramban on Sefer HaMitzvoth, Positive 36). This is actually a more literal translation.