Argentina bus promotions

I am at the end of my 8 week trip in Argentina so it is not much use to me now but I just wondered if anyone knew about the price promotions the bus companies have. It happened to me twice on my trip, once from Cordoba to Puerto madryn I got 20% from Andersmar off but my spanish wasn`t good enough to know why. Then from BsAs to Puerto Iguazu both TigreIguazu and CrucerDelNorte both had 20% discounts that day but when I asked why they just said special promotion.

Is there any way of knowing when these are going to happen? Or if they are more common on certain days?

Obviously bus travel in Argentina is expensive so if there are ways to get it cheaper for future travel it would be useful to know.