The following adverbs can be used to join sentences or clauses. They replace the more formal structure of preposition + which in a relative clause:
where, when, why.
- That's the restaurant where we met for the first time.
(where = at/in which)
- I remember the day when we first met.
(when = on which)
- There was a very hot summer the year when he was born.
(when = in which)
- Tell me (the reason) why you were late home.
(why = for which, but could replace the whole phrase the reason for which)