Well, first we had Thanksgiving. Then we had Black Friday. Then we had the weekend. Then we had CyberMonday. Both Black Friday and Cyber Monday ate big shopping days in the U.S. People were busy - they were not working - they were busy with family and with shopping.
My traffic dropped dramatically for all of those, but it looks like it has come up dramatically since then.
I also think there must have been some kind of system glitch, cause the shift is so dramatic.