The Nine Days!
From May 2nd to 13th.
The French pride themselves on being great at math. Actually, upon closer investigation here, they are perfectly correct. If you count only weekdays and Saturdays (closed Sundays) and account for the May 8th holiday, there are nine days for this sale.
And, so why didn't they write that it ends May 12th, since the 13th is a Sunday?
Don't ask me. It took me awhile to figure this much out.