Worst six months of the year or a great time to buy? There are arguments on both sides, and both have had merit in the past. October is historically the beginning of the better half of the year in terms of stock market returns, and although volatile, on average it produces positive returns. It's the volatility that's hard to stomach.
The graphic below illustrates my point. The box-and-whisker plot shows the aggregated returns, by month, for the past 10 years of the S&P 500. October has two outliers in the ~17% and ~-22% levels of returns, and a much longer whisker on the downside than upside. Based on this graphic alone, October may look like a month to buckle up, strap in for the ride and build positions for the remainder of the year.
With this volatility in mind, let's take a step back and compare the S&P 500 price chart of today (in red) to the historical price chart over the period of Jan, 3, 2003 - Jan 2, 2009. Of course there is no guarantee that previous returns and actions will influence future returns or actions, but there is more than a coincidence in the two time frames. Given the volatility (and a strong dose of superstition), I'm going to tread lightly on the long side.
Disclosure: I have no positions in any stocks mentioned, but may initiate a short position in SPY over the next 72 hours.