click for larger chart
Source: Bianco Research
The chart above shows a composite of market returns by month since 1981.
This is merely a rough overview, and does not have much in the way of predictive value. What we can say is that on average, December has been a pretty good month for equities the past few decades.
Some folks disagree that this December is going to continue that tendency. Se e.g., MarketBeat: December Rally No Sure Thing.
What's been said:Discussions found on the web: