Been diving into some historical stock market performance data and found something pretty interesting about how the S&P 500 actually behaves month to month. Turns out the conventional wisdom about 'selling in May' is basically nonsense - the index has historically done fine through summer, with July being one of the strongest months overall. What caught my attention though is how seasonal patterns show up when you look back decades.

I grabbed data spanning nearly a century of historical stock market performance by month, and the pattern is wild. The market's up about 9 out of 12 months on average, but here's the real eye-opener: September consistently shows weakness. Like, really consistently. But then it bounces right back when people get excited about the holidays. If you've got cash sitting around, that September dip might actually be an opportunity worth watching.

The thing that actually matters though - and this is what most people miss when they obsess over monthly trends - is that the longer you hold, the more the monthly noise disappears. Over any rolling 20-year period since 1928, the S&P 500 has always been profitable. Always. That's not luck, that's just how long-term equity investing works. Compare that to other asset classes and the historical stock market performance by month gets even more interesting because this index has outperformed bonds, real estate, commodities, and international equities over the past several decades.

The real lesson from studying historical stock market performance by month isn't about timing individual months - it's realizing that short-term volatility is just noise if you're thinking in decades. Monthly returns vary wildly, but zoom out and the picture becomes crystal clear. Most people would be better off just staying invested rather than trying to play seasonal games.