Using When

When is used in asking questions.
–When are you leaving?


When can also be used in a dependent clause:
–I forgot the date when you arrived.


In the latter example, when is used to refer to a noun of time (i.e., a day, week, month, etc.)


In time clauses, it is also possible to use that or which preceded by a preposition:

–I forgot the date that you arrived.
–I forgot the date on which you arrived.


No preposition is needed with that.


Note how two sentences are combined using when:


–I'll always remember the day she was born. She was born then (on that day).
–I'll always remember the day when she was born.