Look at these sentences (a) She sat in the corner of the room where the rough boys who did not make good marks sat, the corner of the room where there was most scuffling of feet. (b) The time when they thought about Wanda was outside of school hours. These italicised clauses help us to identify a place, a set of boys and a time. They are answers to the questions, 'What kind of rough boys?' 'Which corner did she sit in?' and 'What did they think of Wanda'? They are 'defining' or 'restrictive' relative clauses.
Combine the following to make sentences like those above.
You must break your fast at a particular time (When?) You see the moon in the sky. (use when)

Explanation

You must break your fast when you see the moon in the sky.