Forget, the Federal Reserve interest-rate hike. The 37th week of the year has historically been a winning stretch for stock-market investors While the focus is on the Federal Reserve and what it’ll be doing to interest rates, history tends to smile on the 37th week of the year. Of course, September is known to be an abysmal spell for stocks, but recent performance suggests that this particular week is a big winner. In fact, it is the best week of the year, according to blogger Ryan Detrick. Over the past decade, the next five days have turned in a positive S&P 500 performance nine times, with an average return of 1.7% to lead all other weeks. But before you back up the truck, beware of what happens in the weeks that follow. They live up to September's reputation. Check it out in the attached illustration: But bullish seasonality is butting up against another trend, says Detrick. The S&PSPX, +1.28% has alternated between higher and lower for 10 weeks in a row, which has only happened eight times ever. Last week was green. So one trend is going to break down, and by the looks of Monday’s trade, it could be red’s turn. More from MarketWatch