Relative Adverbs :
Adjectival clauses are sometimes introduced by what are called the relative adverbs: where, when, and why. Although the entire clause is adjectival and will modify a noun, the relative word itself fulfills an adverbial function (modifying a verb within its own clause).
The relative adverb where will begin a clause that modifies a noun of place:
My entire family now worships in the church where my great grandfather used to be minister.
The relative pronoun "where" modifies the verb "used to be" (which makes it adverbial), but the entire clause ("where my great grandfather used to be minister") modifies the word "church."
A when clause will modify nouns of time:
My favorite month is always February, when we celebrate Valentine's Day and Presidents' Day.
And a why clause will modify the noun reason:
Do you know the reason why Isabel isn't in class today?
We sometimes leave out the relative adverb in such clauses, and many writers prefer "that" to "why" in a clause referring to "reason":
Do you know the reason
why Isabel isn't in class today?
I always look forward to the day
when we begin our summer vacation.
I know the reason that men like motorcycles.