The promotions are by invitation only. If you have not received the details and provided your promotion, your offers will not be included. I think the deadline is Friday.
I suggest that you do not be too concerned about it though. You can offer your own promotions or price reductions that will compete. Buyers can not see every Amazon deal, anyway and there will be lots of buyers looking for deals.
Remember that they are looking for deals, that means low margins on every sale. I prefer to keep my inventory for those who are not as keen for deals to make more profit on each sale.
> As sellers, do you do anything special for Black Friday or Cyber Monday?
I price some items I want to clear attractively, but keep my popular items for afterward. Why sell out your inventory for a 5% profit (or less) when you could sell it later for a better profit?
> If you have not done any promotions, do sales increase on these days? Should we expect to send more stock to cover those days even if we do not promote specifically on those days?
Sales definitely increase for the entire weekend. If you want to sell during that time, be prepared with some extra inventory or be prepared to restock soon after. Christmas is still coming and you will still make more on the days leading up to Christmas than on that weekend.