The rules are slightly more complicated.

You are only allowed to discount items already in stock a month before the start date[0] That's why the back of the shop is fully loaded by boxes a month before the sales period starts.

The rules seem silly because it's an open secret, all companies jack up the price and stock up just before... but you'd be hard pressed to find actual proof:

"I ordered 50k items to the supplier just 30 days before, I meant to buy only 50, fat finger mistake, so we had to discount them".

"Yes, the reference price is based on the highest price we had on record among our 500 stores. Yes, I know it was more than twice the median price, fat finger mistake but hey, who does not make mistakes, 14€99 is too similar to 5€50..."

[0]https://www.service-public.fr/professionnels-entreprises/vos...

True in some forms (in the Netherlands) but if you only sell it for a day for the increased price you could argue it is deceptive (Mediamarkt does this usually for a week before the sales which is apparently enough). What they used to do, raise the price on the day of the sale, is fortunately no longer allowed. The other trick being the use of the advised selling price (which is always way more than the actual value of the product) and go from there.

What I usually see these days is that products get offered on sale which have not been sold before so there is no way of knowing if they offered it before for less. Though internet helps if you are in a shop and it sounds dodgy at best.