Last year the Black Friday sale started on the Friday of CS morning and lasted about 5days.. hoping it'll be more days this year too.
Almost everything was offered on sale, older ex-event buildings, ex-decos, various dragons, animals, gold mines, cabbage hut, daily Alis, all kinds of weekly Alis, weekly Horses.. and other things.
Gems were put on sale at times during this period but with less discount (if any) than are usually offered.
Hope that helps.