Based on the experience of last July until now. Some months sales allow only one product per seller and some month they allow you to put multiple products on sale.
Also some months, regardless of number of products allowed, you can choose between multiple discount options (usually 30%, 50% and 70%) and some months one of these is chosen as a fixed price.
Most sales are one week but for example the black friday one was 2 weeks IIRC.
I have not been invited to theme based sales yet.