A relative adverb is an adverb which introduces a relative clause. The English relative adverbs are when, where, whenever, wherever and why.
I will never forget the day when I first met you.
Sunday is the day when I am least busy.
Do you know the house where John lives?
That is the place where I was born.
Whenever I go to Paris I try to see Peter.
You can come whenever you like.
I take my cellphone wherever I go.
Whenever I see you I feel nervous.
Wherever you go, you will find Coca-Cola.
We usually use when and where to introduce relative clauses after nouns referring to time and place. They are used in the same way as preposition + which.
I will never forget the day when I first met you. = I will never forget the day on which I first met you.
Sunday is the day on which I am least busy.
This is the place at which I was born.
Why can be used in a similar way after the noun reason.
I don’t know the reason why she hates me. (= I don’t know the reason for which she hates me.)