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.
Please use the comments to demonstrate your own ignorance, unfamiliarity with empirical data and lack of respect for scientific knowledge. Be sure to create straw men and argue against things I have neither said nor implied. If you could repeat previously discredited memes or steer the conversation into irrelevant, off topic discussions, it would be appreciated. Lastly, kindly forgo all civility in your discourse . . . you are, after all, anonymous.