Halliday & Matthiessen (2014: 466-7):
Clauses with when or where, having as domain some expression of time or place, e.g.
||| The first few days are a time for adjustment, || when the kitten needs all the love and attention [[ you can give it]] . |||
The meaning is ‘which is when . . .,’ ‘which is where . . .’. Those with where often refer to abstract space, as in
||| Now consider the opposite situation, || where the velocity decreases. |||
In this group also the secondary clause may be enclosed, as in
||| One evening, << when the boy was going to bed, >> he couldn’t find the china dog [[ that always slept with him]] . |||
As in the examples above, such clauses often elaborate marked Themes of time or place. In addition to when and where we also find elaborations of temporal expressions introduced by as, when; for example:
||| That night, << as Kukul slept on his straw mat, >> Chirumá came upon him. |||
As the examples illustrate, this strategy of a nominal group denoting a time plus a hypotactically elaborating clause is common in narratives when the time is being set or reset in the episodic sequence of events.