You don't have my choice: Almost Never.
I did attempt one just today, in fact. A friend emailed me a link to one entitled "Geek Crossword #11 - Monty Python & The Holy Grail".
I am sad to say that my Holy Grail aptitude was less than I thought. OTOH, this means that I'll have an excuse to watch the movie again
For those who wish to attempt it: http://ars.userfriendly.org/cartoons/?id=20090531