So I was digging through some historical market data and found something interesting about stock performance patterns. Turns out certain months have consistently underperformed over nearly a century of S&P 500 trading.

Most people don't realize this, but since 1928, there are actually only three months that have produced negative average returns. September is the real problem child here - it's averaged a 1.1% loss. February and May both show slight declines too, around 0.1% each. When you look at how many times these worst months for stock market actually closed in red, September takes the crown with 52 down closes versus just 42 positive ones since 1928.

The interesting part is why this happens. February and May weakness typically follows the strong runs in December and January, so profit-taking kicks in. September's rough performance might be connected to summer vacations ending and traders returning to lock in gains from the lighter summer trading volume.

Now here's where it gets good - on the flip side, you've got the consistent winners. July absolutely dominates with an average 1.7% return when it closes positive. April comes in at 1.4%, December at 1.3%, and January at 1.2%. The December-January combo is what they call the Santa Claus rally, and it makes sense given year-end sentiment.

But here's the thing that actually matters more than tracking worst months for stock market - and this is backed by serious data from Crestmont Research. They looked at every possible 20-year rolling period in S&P 500 history going back to 1900. Out of 104 different 20-year windows, every single one generated positive returns. That's a 100% success rate. Whether you bought at a bear market bottom or jumped in at a local peak, holding for two decades would've made you money.

So while it's useful to know September historically struggles and July tends to shine, the real takeaway is that short-term monthly patterns barely matter. The traders obsessing over worst months for stock market are missing the bigger picture. Long-term investors who just stayed put for 20 years never lost money on the S&P 500. Not once in over a century of data.

That's the closest thing to a guarantee Wall Street will ever give you.