Just dug into some historical stock market performance by month data going back to 1928, and there's some interesting patterns that most people miss. Turns out the S&P 500 was actually profitable in 9 out of 12 months historically - way better odds than people think. But here's the thing: if you zoom out, the real edge comes from holding longer. Monthly returns are basically a coin flip at 59%, but stretch it to 20 years and you're looking at 100% win rate across every rolling period since 1928.

The September effect is legit though. That month gets absolutely hammered while summer months like July are consistently strong, which completely kills that old 'sell in May' wisdom. What's wild is how the historical stock market performance by month shows that most of the real wealth building happens when people just stay invested. Looking at the data, owning an S&P 500 index fund for two decades straight has never lost money in any 20-year window. Pretty solid reminder that time in market beats timing the market.