Like relative pronouns, relative adverbs can be used to join two clauses together. The relative adverbs when and where can introduce relative clauses after nouns referring to time and place.

This is the house. The theft was committed here.
This is the house where the theft was committed.

His father died. At that time he was only six.
At the time when his father died he was only six.

In most cases, a preposition + which can be used instead of when and where.

This is the house at which the theft was committed.
I will never forget the day on which I first met Susie. (= I will never forget the day when I first met Susie.)

Why can be used in a similar way after reason.

You refused to sing. Can you tell me the reason?
Can you tell me the reason why you refused to sing?

She doesn’t like me. Do you know the reason?
Do you know the reason why she doesn’t like me?

Instead of why, we can use a preposition + which.

Can you tell me the reason for which you refused to sing?
Do you know the reason for which she doesn’t like me?