Between the last week of December and the first week in January, Bloomberg reports that the S&P 500 has reversed direction every year since 2011.
A possible explanation is the expiration of government policies on Dec. 31.
After 4 trading days, the S&P 500 is up 2.6% year-to-date - that is the best start to a year for the S&P since 2006 (and would have been the best weekly gain in 2017).
Critically though, as Ryan Detrick notes, "since 1950, when the first 5 days are up over 2%, the S&P 500 is higher for the year 15 out of 15 times with an average return of +18.6%. "
So, as Detrick concludes, "if you are bullish, Monday is a big day."
Of course, January is another seasonally strong month, with an average total return of +1.2% (+1.1% price return) since 1928. A positive January has historically led to a positive year 86% of the time (80% on a price return basis), with an average total return of +17% (+13% price return). A down January has led to a negative year 47% of the time (56% on a price return basis) with an average total return of +2% (-1% price return).