The Special Offers page of the website will show any available offers or discounts. I don’t recall seeing anything like a Black Friday sale in the past. Sometimes when new itineraries or dates drop, select departures will have a small discount for an initial period of time.
They tend to discount trips right after trips are announced in May or early June. They also might run last minute sales to fill spots, but those aren't consistent.
Generally, the cheapest prices are when trips are released. Land trips for next year go on sale around May, and river cruises early in the year (if I remember correctly). ABD does dynamic pricing so prices go up as trips fill. They are opaque about how they do this, but I think the lowest prices are only for the first couple of guests. This is why there can be long waits on the phone on the first days that new trips go on sale. Or you can have a travel agent book for you (Dreams Unlimited sponsors this board).
It's common to see ongoing discounts for certain trips. Right now their "Last Chance Offer" provides $200-400 discounts for certain trips.
I haven't seen discounts tied to particular days such as Black Friday.
If you are a DVC member, I think you always get opening-day prices even if prices have gone up.