When are you going to leave on vacation?
When I arrive there, I`ll send you a postcard.
When the telephone rang, I was having a shower.
when - adverb:
at what time or period? how long ago? how soon?: When are they to arrive? When did the Roman Empire exist?
when - conjunction:
Whenever: When the wind blows, all the doors rattle.
During the time at which; while: When I was young, I was sick all the time.
when - Definitions from Dictionary.com