I ran a custom report in Google Analytics looking at Pageviews and Sessions - by day - for the last year. For the most part, the daily range of Sessions is between 50,000 and 90,000 (never once breaking 100k). Yet on December 6, 2013 - a very abnormal number of 590,650 Sessions appear (with 577,476 Pageviews). Does anyone know if Google might've been doing something around that day? Or why such a huge spike would occur?
Turns out it was a bad bot from amazonaws.com. Looks like Google's defenses might've been down that day and the bot got through...