Black Friday is the Friday after US thanksgiving and is considered to start of Christmas shopping although here Christmas stuff has been out for over a month. Many Canadian stores also have big sales at the same time. Lots of apps in the US and Canadian stores also go on sale. Not sure if it extends to other countries. I always make sure I have a decent credit in the iTunes Store so if I find something....
I normally buy iTunes gift card when they come on sale for the 20% off! that way being a AppWhore doesn't cost me as much although I have bought lots of apps at full price when I didn't have a credit