Students who know my little vocabulary tests will remember that I keep telling them to avoid the construction "...is when..." at all costs when explaining a piece of vocabulary in English. As some will just as repeatedly point out, it is, of course, theoretically possible to use said construction in a sentence WITHOUT making a serious stylistic blunder. So, for all you advanced English students out there, here's how to do it RIGHT...
Consider the following example:
"Halloween is when many people put hand-carved Jack-o-Lanterns in their front yards." (perfectly correct)
as opposed to
"A cat is when it makes 'meow'". (abysmal)
See the difference? The first sentence actually refers to a time ("Halloween"), as does the relating conjunction "when". "A cat", on the other hand, is not a way of telling the time at all (imagine someone answering the question of "When shall we meet again?" by saying "cat"...)! In short:
Only use "is when" if you want to talk about time!
(In fact, it's still the safest bet NOT to use it at all, I should say.)