Key Points

• The S&P 500 reached a new intraday record during the Christmas Eve trading session.

• December 26 has historically been the S&P 500’s most reliably positive day, with an average gain of 0.5% over 39 years.

• The ‘Santa Claus rally’ has yielded negative returns for the past two years.

Stocks were riding high ahead of the Christmas holiday on Wednesday, with the S&P 500 tallying a fresh intraday record during the abbreviated Christmas Eve session.

But for investors, the real gift from Wall Street comes the day after Christmas.

For the S&P 500 SPX, Dec. 26 has historically been the most reliably positive day of the year, according to an analysis from Bespoke Investment Group. During the 39 years since 1953 that the market has been open on the day after Christmas, the index has only declined six times, never falling more than 0.5%.

When the market is open, the Dec. 26 session has also seen the index's largest average - 0.5% - and median - 0.4% - gains of the entire calendar year.

"Seasonal trends should never be used as a sole reason for going long or short the equity market, but that's pretty consistent!" the Bespoke team said in commentary shared with MarketWatch.

This year, Friday's session will mark the second day of the "Santa Claus rally" period. The stakes for the rally are particularly high this year: The seven-day stretch that includes the last five trading days of one calendar year and the first two of the next has produced negative returns during the past two years.

Never before has Santa failed to come three years in a row, Dow Jones Market Data showed.

U.S. stocks were trading higher on Friday, with the S&P 500 up 0.2% at 6,922 in recent trading. The Nasdaq COMP was sitting on marginal gains, while the Dow Jones Industrial Average DJIA was doing the best of the three major U.S. indexes.