Loads of times at school - but the m ost memorable was when I was 11 and had to sing for the first time at Covent Garden.
No - it wasn't anything grand - merely that our school used to be in the pool that provided kids for juvenile choruses in their operas, and I got picked a few times for a chorus part.
My abiding memory of the place was the canteen, where these hefty cockney women (whose husbands were generally working in the fruit and veg market) would serve up vast portions of egg and chips and fuss over us like mother hens.
The first opera for us was Taverner by Peter Maxwell Davies - and to an 11-year-old it sounded 'orrible. We had to learn a chorus in Latin to be sung while we capered across the stage, half naked, covered in red, white and black bodypaint and wielding rubber tridents.
As we came on, the male lead would be taking apart a life-sized statue of the Virgin Mary and removing her entrails - lengths of cloth-covered foam rubber like a giant string of sausages.
Offputting? Only the first time, and thereafter we just treated the roles as fairly routine; preferring the larking around backstage and the canteen to the performances themselves. I only saw the whole opera thorugh once, and that was because I chose to and could arrange to be picked up at the end by my parents - otherwise we arrived after school, got costumed and painted, stuffed our faces and then did five minutes on stage before trooping off, scrubbing up and going home.
The biggest problem was a few of the members of the male (adult) chorus, who would find pressing reasons to visit the showers when we tried to scrub off the body-paint (the red stuff stained for days and made us look as if we'd been beaten). Then the battleaxes from the canteen would spring into action, emerging from where they'd been keeping a motherly eye on us and ordering the chorus boys out of the showers - "g'warn, piss off you old pooves! Honestly - leave the bleedin' kids alone!".
We were ferried everywhere by a minibus driver with one leg and the most awesome command of filth and invective even I had seen (and my father was legendary). Driving to and from the opera house we'd be kept entertained by his opinions of every other driver on the road and his inept crashing through the gears like a rhino with Tourette's. Someone was carsick every journey, so we'd always have the windows open, heads sticking out and abusing passers-by while Tony the driver told the angelic choir of 11-year-olds to "F***ing sit down, yer little c***s, or I'll cut yer b******s off wiv a rusty breadknife!"
Highlight of the Taverner run was that Radio3 broadcast it, so I ended up with a cheque for £15. Otherwise it was all done for love and the fun of it - and because Miss Povey had said we should regard it as a great honour (though I suppose the school trousered a fee for providing us lil' darlings).
Other operas we did included Turandot and Carmen, but none was as fun as the awful Taverner.

And then my voice broke, and now I sing like a file crossed with a fog-horn. No more Covent Garden for me!