in which === where
I went to the park in which I walk my dog.
I went to the park where I walk my dog.
whose === [ of which.{things only}]
The book whose cover is green is mine.
The book of which the cover is green is mine.
*** when === in/ on /at + which
I know the time when he will come.
I know the time on which he will come.
at which === where
The hotel at which we stayed was very comfortable.
The hotle where we stayed was very comfortable.
I still remember the day + [when /on which /that]
+ we met for the first time.
*** when ===on the day
*** which (N)===the day,on which ===on the day === that