when

1. at what time.

   when did you last see him?

   since when have you been interested?

1. at or on which (referring to a time or circumstance).

   Saturday is the day when I get my hair done

1. at or during the time that.

   I loved maths when I was at school

2. after which; and just then (implying suddenness).

   he had just drifted off to sleep when the phone rang

3. in view of the fact that; considering that.

   why bother to paint it when you can photograph it with the same effect?

4. although; whereas.

   I'm saying it now when I should have told you long ago

Old English hwanne, hwenne ; of Germanic origin; related to German wenn ‘if’, wann ‘when’.