“Moody, we never use Transfiguration as a punishment! ...Surely Professor Dumbledore told you that?”
-- Minerva McGonagall (GF13)
After insults about mothers are thrown back and forth Draco Malfoy tries to hex Harry, but is turned into a ferret by Moody. Moody gets into a bit of trouble with McGonagall as a result.
We know that this is also a Monday from this reference at breakfast that day:
"'Today's not bad.. . outside all morning,' said Ron, who was running his finger down the Monday column of his schedule."
UPDATE: This error in the text was fixed in later editions (c. 2004).