Relative Adverbs

Relative adverbs are when, where, and why. They are used in adjective clauses in a similar way to relative pronouns (who, whom, that, and which).

Using WHEN in Adjective Clauses

When is used in adjective clause which modifies the time noun. It replaces a preposition + noun (time). Look at the examples.
1. I never forget the day. I met you then (on the day).
→ I never forgot the day when I met you.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'when' is used instead of 'preposition + noun'.
→ I never forgot the day which I met you on.
→ I never forgot the day on which I met you.

2. Noon is the time. We eat lunch then (at the time).
→ Noon is the time when we eat lunch.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'when' is used instead of 'preposition + noun'.
→ Noon is the time which we eat lunch at.
→ Noon is the time at which we eat lunch.

3. I was born in the 1980s. Our country was so poor then (during the time).
→ I was born in the 1980s when our country was so poor.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'when' is used instead of 'preposition + noun'.
→ I was born in the 1980s during which our country was so poor.

Using WHERE in Adjective Clauses

Where is used in adjective clause which modifies the place noun. It replaces a preposition + noun (place). Look at the examples.
1. The building is very old. He lives there (in that building).
→ The building where he lives is very old.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'where' is used instead of 'preposition + noun'.
→ The building which he lives in is very old.
→ The building in which he lives is very old.

2. The city was beautiful. We spent our vacation there (in the city).
→ The city where we spent our vacation was beautiful.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'where' is used instead of 'preposition + noun'.
→ The city which we spent our vacation at was beautiful.
→ The city at which we spent our vacation was beautiful.

3. I went to the village. I was born there (in the village).
→ I went to the village where I was born.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'where' is used instead of 'preposition + noun'.
→ I went to the village which I was born in.
→ I went to the village in which I was born.

Using WHY in Adjective Clauses

When is used in adjective clause which modifies the time noun. It replaces a preposition + noun (time). Look at the examples.
1. He didn't tell us the reasons. He left the company for the reasons.
→ He didn't tell us the reasons why he left the company.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'why' is used instead of 'preposition + noun'.
→ He didn't tell us the reasons for which he left the company.

2. Do you know the reason? She cried because of the reason.
→ Do you know the reason why she cried?
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'why' is used instead of 'preposition + noun'.
→ Do you know the reason because of which she cried?

3. Tell me the reason. You came home late due to the reason.
→ Tell me the reason why you came home late.
Alternatively, you can use 'which' to join these two clauses, too. Remember that 'which' is used instead of a noun, while 'why' is used instead of 'preposition + noun'.
→ Tell me the reason due to which you came home late.

Reference

Betty Schrampfer Azar (2002). Understanding and Using English Grammar. Third Edition. Pearson Education.