Why 2015 could have this rare market ending

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On Thursday we may all get to live through a truly momentous market event.

The S&P 500 is hovering close to 2,058.90, the close at the end of the last year. We could land exactly on that number and break even exactly for the year. That has only happened once since 1929 — in 1947. The S&P 500, in its current form, launched in 1957, but S&P uses historical data to track how the index would have performed going back to the 1920s.

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If we close Thursday between 2,038.31 and 2,079.48, that would put us less than 1 percent away from last year's close. That's only happened four times since 1929, including most recently in 2011 when the difference was -0.0025 percent.

So if the S&P 500 lands on the same exact number Thursday, you'd be witnessing a true rarity during your professional career. The only time we hit exactly on zero percent change for the year was in 1947, when the reconstructed S&P 500 ended at 15.30 that year vs. 1946.

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The most common windows for a year's return are between 10 and 15 percent and -10 and -15 percent. More than a fourth of yearly returns since 1929 have fallen within those two 5 percentage point windows, and only 14 years have been within 5 percent of last year's close. So even if we only get close, it will be an unusual year.

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If nothing else, this unique event will make for good office betting on the last trading day of 2015.