To this point in Wednesday's session, stocks look to be finishing September in fine form, but even Wednesday's impressive rally will not be enough to erase the memory of what has been a dismal month for U.S. equities.

September's reputation for being unkind to stocks was reaffirmed this year with the S&P 500 positioned for a September decline of just over 3 percent. Although October has had a penchant for bringing some of the darkest days in U.S. equity market history, the S&P 500's last 20 October performances, when averaged out, are actually quite sturdy.

Twenty-Year Review

Over the previous two decades, the S&P 500 has posted an average October gain of 1.8 percent, while rising in 65 percent of the time in the tenth month of the year, according to EquityClock.com. Perhaps unbeknownst to many investors is that over those 20 years, the S&P 500's October gains were superior to those notched in January and December.

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Looking At Specifics

XLK, the largest technology ETF, posts an average October gain of just over 3 percent, according to CXO. If the ETF upholds that October seasonality this year, it will likely be because investors decided to revisit shares of Apple Inc. (NASDAQ: AAPL) because the iPhone maker is XLK's largest holding at a weight of 16.35 percent. That is 722 basis points more than XLK devotes to its second-largest holding, Microsoft Corporation (NASDAQ: MSFT).

In what could be seen as a sign of investors warming up for the holiday shopping season, the Consumer Discretionary SPDR (ETF) (NYSE: XLY) is historically the second-best SPDR in October, averaging a gain of just over 2 percent, according to CXO data. Entering Wednesday, XLY was the only one of the nine SPDRs sporting a year-to-date gain.

On an anecdotal basis, what is interesting about the leadership of XLK and XLY in October is those ETFs rarely rank among the top two SPDRs through the other 11 months of the year. In fact, XLK only commands that distinction in one other month, January, while October is the only month where XLY is one of the two best sector SPDRs.

October's Historically Ghoulish SPDRs

As for the SPDRs to avoid in October, none of the nine average negative returns, but the least good are the Utilities SPDR (ETF) (NYSE: XLU) and the Health Care SPDR (ETF) (NYSE: XLV).

With another Federal Open Market Committee meeting coming in October, it makes sense to approach the rate-sensitive XLU with caution. As for XLV, the largest healthcare ETF may have gotten a head start on lagging in October by tumbling more than 6 percent in September.