There is a Black Friday on every Saturday. On that day, masses of players line up inside every town, and some just camp out in front of the castle, for a chance to trample, script, trash talk, and get a feel good ego boost and try to get that special good deal on a castle.
If they do anything it'll most likely be an Ncoin sale or certain items on discount and it'll only probably run 24 hours. Although they haven't hinted at any such sale that I've seen so can't say for sure.