Rosh Hashanah the Jewish New Year begins tomorrow at sundown. Decades ago, market watchers advised buying on that particular holy day and selling on Yom Kippur, the solemn Day of Atonement that closes the Ten Days of Repentance commencing with Rosh Hashanah.

But as David Gaffen points out in today's MarketBeat blog at, this trend has shifted over the past few decades. In fact, Gaffen notes: What's working now is the reverse selling on Rosh Hashanah, buying on Yom Kippur the idea being that more people are closing out positions in advance of spending the holidays with family.

From 1971 to 2005, the S&P 500 Index (SPX) has dropped an average of 0.4% between the 2 high holidays. In 2005, the index fell 2.2%; it dropped 1.9% in 2004.

Meanwhile, there's the traditional wisdom that September is the worst month to be invested, but that has been challenged in recent years as well. And depending on what the Fed decides to do (or not do) a week from today, all bets based on the calendar could be off.