Have you already checked the websites directly, or do you prefer waiting for an official email? Anyway, the answer is what the others said - end of November, probably around the 24th - 25th - but the real trick is that they start with "pre-Black Friday" stuff as early as October/November and gradually ramp up the discounts, so it's not like everything kicks off together on the 24th. It's a strategy so they don't lose customers who are already starting to look around.
What experience has taught me, though, is that if you wait for the actual peak (last days of November into early December) you often find more stuff with decent discounts, especially items that don't sell out fast. Last year I waited until almost the end of November to grab a couple of things at Mediaworld and I saw the price drop even days after Black Friday had "officially" started.
Sign up for notifications on their websites, so they'll tell you directly when the real deal starts. Mediaworld and Unieuro send emails even a week before, so you don't have to keep checking the web constantly.