Relative when, where and why

When and where can introduce relative clauses after nouns referring to time and place. They are used in the same way as preposition + which.
I'll never forget the day when I first met you. (=... the day on which...)
Do you know a shop where I can find sandals? (=... a shop at which...)

Why is used in a similar way after reason.
Do you know the reason why she doesn't like me? (=... the reason for which...)

Taken from Practical English Usage by Michael Swan