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On friday: lowest traffic and very low clicks. I think everyone does some christmas shopping instead of surfing. It seems to become even worse today.
The same thing happened on the Friday before Christmas week last year: My unique visitors, page views, AdSense impressions, and AdSense revenues fell noticeably. There was a big recovery after the first of the year.
Unless you're selling Christmas-gift items or running a site about Santa Claus, the traditional holiday lull is nothing to worry about. :-)
That said, I noticed a slight change in poorer targeting Saturday too (more general, apparently lower bids). I've noticed sudden changes (like in algorithm) almost every Saturday, sometimes good, sometimes not.