Relative adverbs: WHERE, WHEN, WHY

The relative adverb where is used after nouns that refer to places:

The house where Mozart was born is now a museum. (defining relative clause)
I flew to Munich, where I had to catch another plane to Oslo. (non-defining relative clause)

The relative adverb when is used after nouns that refer to times and dates:

I can't remember a time when I was so happy. (defining relative clause)
The most stressful day of the week is Monday, when people go back to work. (non-defining relative clause)

The relative adverb why is used after reason:

The reason why I didn't call you is that I've lost your phone number. (only in defining relative clauses)