Been digging into some historical stock market data and noticed something interesting about average monthly stock market returns. Turns out the S&P 500 isn't equally profitable every month - there are definite seasonal patterns if you look back far enough.

So here's what caught my attention: Over the past century or so, the market actually went up in 9 out of 12 months on average. But here's the kicker - September has historically been rough. The index tends to drop noticeably that month, though it bounces back hard in October, probably because people get excited about the holiday season. Meanwhile, that old saying about selling in May and disappearing? Total myth. Summer months are actually pretty solid, with July being one of the best months historically.

What really matters though is the holding period. Looking at the average monthly stock market returns is interesting, but it's kind of noisy. The real insight is when you zoom out. On a monthly basis, you're profitable only about 59% of the time - basically a coin flip. But stretch it to a year and your odds jump to 69%. Five years? 79%. Ten years? 88%. And here's the wild part - over every single 20-year rolling period since 1928, the S&P 500 has never had a negative return. Never. That's a pretty compelling reason to think long-term.

The S&P 500 tracks 500 large U.S. companies representing about 80% of domestic market cap, so it's basically the heartbeat of American stocks. Over the past three decades alone, it returned about 1,710% total, compounding at roughly 10% annually. When you account for different market cycles - booms, recessions, everything - that kind of consistency is hard to argue with.

Bottom line: Average monthly stock market returns vary wildly, but if you're actually holding for decades, the data suggests you're going to be fine. That's why so many people just stick with an index fund and forget about trying to time things.