By Jason Zweig and Tom McGinty

The Jewish High Holy Days are upon us, with the festive New Year celebration of Rosh Hashana beginning at sundown today and the solemn day of repentance, Yom Kippur, beginning Friday, Sept. 13. So it's worth looking at a popular old market belief: "Sell Rosh Hashana, Buy Yom Kippur," often abbreviated in traders' lingo to "Sell Rosh, Buy Yom."

Naturally,...