Relative adverbs act as subjects or objects inside relative clauses, and at the same time they connect relative clauses to nouns or pronouns in other clauses – rather like conjunctions. The relative adverb when means “in which” or “at which” and is used to refer to a time expression. Noon is the time when we eat lunch. The relative adverb why means “for which” and is used to refer to a reason. I don't know the reason why Larry isn't in class today. The relative adverb when can be replaced by ‘in/on which’. Where can be replaced by ‘in/at which’. Why can be replaced by ‘for which’.
A relative adverb is an adverb that introduces an adjective clause.
Where is an adverb of place.
When is an adverb of time.
Why is an adverb of reason.