Twice a year there is a major sale “les soldes” during 6 weeks. The wintersale starts the first wednesday in January and the summersale end of June in Paris. People travel from all over the world to Paris specially for this sale.
The french law tightly regulate when stores can and cannot heavily reduce their prices. However the french gouvernement is a bit more flexibel now, the retailers use these period to put everything in promotion even on designer items. A discount up to 80% is not an exception.
Surviving the sales requires strategy, patience, and lots of energy! On a regular weekend the shops and big departement stores are overcrowed and queuing for half an hour is nothing. So imagine during the sale. To avoid getting pushed out of the way by impatient fellow shoppers, the best timing is during the week or early morning.