Been digging into some historical stock market performance by month data, and there's some interesting patterns most people miss. So the S&P 500 has returned something like 1,710% over the past 30 years, averaging around 10% annually. That's solid, but here's what caught my attention - the index actually goes positive in 9 out of 12 months historically. Not bad odds.

The monthly breakdown is pretty revealing though. Everyone talks about 'sell in May and go away,' but that's honestly outdated advice. June through August actually tend to be decent months, and July has been one of the strongest historically. September is the real problem month - it's consistently weak - but then things bounce back hard in October and November. Probably holiday spending momentum kicking in.

Here's the thing that really matters: if you look at the odds of making money based on holding period, it changes everything. Monthly returns? Only 59% positive. But stretch it to 5 years and you're at 79%. Go 10 years and it's 88%. Twenty years? The S&P 500 has literally never been down over any rolling 20-year period since 1928. That's 1,152 months of data we're talking about.

So the real lesson from historical stock market performance patterns isn't about timing individual months. It's that the longer you hold, the more the odds shift in your favor. The S&P 500 has outperformed pretty much every other asset class - international stocks, bonds, real estate, commodities - over the last 5, 10, and 20 years. Makes you think about what actually works for building wealth long-term.