There are not any "Christmas specials" that I know of. Some prices go up the first of each year, but that is mostly Italian guns since they raise their prices most years.
HOWEVER, Cimarron is currently out of 1878's. If you call them they may be able to tell you when a shipment is expected. It could be shortly or it could be a long time. There have been some fairly lengthy dry spells, but then they get a big shipment (usually at least 600 guns).
But there are a lot of them out there in various dealers hands, so finding one should not be hard. Try contacting OklaTom or John Barleycorn through this site.