"Mum found this stack of order forms when she was cleaning Fred and
George's room," said Ron quietly. "Great long price lists for stuff
they've invented. Joke stuff, you know. Fake wands and trick sweets,
loads of stuff. It was brilliant, I never knew they'd been inventing all
that . . ." "We've been hearing explosions out of their room for ages, but
we never thought they were actually making things," said Ginny. "We
thought they just liked the noise."
No mention of where and when in this quote.
But I'd wager they were doing magic in their room. It's a safer bet
than the one they risked in GoF.
"Accio! Accio! Accio!" she shouted, and toffees zoomed from all sorts
of unlikely places, including the lining of George's jacket and the
turn-ups of Fred's jeans. "We spent six months developing those!"
Fred shouted at his mother as she threw the toffees away.
This actually prooves that they don't make jokes and candy exclusively
during the summer holiday, because it doesn't last six months.
Good thinking! That does take care of the apparent conflict between their
jokes and the restriction on underage wizardry after starting school.
But it is possible that even these two respect the lows on underage
wizardry. In actuality, this is most likely a case of selective low
enforcement, as unfair as that may seem, the muggleborns would be even
more upset if they were the only ones required to take hame the letter.
And it's much easier just to magicopy and distribute one for each kid
in the school.
I bet that Draco can do whatever he wants in the summer!