Looking back, I see a big dip in Google traffic on 11/21-22 on my Penguinized/Pandalyzed site, but then the traffic shoots up on 11/23 to a modest increase which has been holding ever since. But I don't see any shifts around the 27th. My site is informational - are those of you who saw shifts on the 27th all ecommerce? If so, maybe there was a separate algo update that affected ecommerce sites?
It also occurred to me that the 27th was the day after Cyber Monday, so there probably was a huge drop in online shopping that day. But (a) that doesn't affect every ecommerce site, such as B2B which buys according to fiscal events rather than holidays and (b) if that's all it was, you'd be seeing a drop in ALL search traffic, and I'm assuming you all checked into that and found it was just Google.