This Great Graphic shows how different measures of US equities perform in December by day for the past 20 years. I got it as a tweet from Urban Carmel, who got it from the Stock Almanac.  

Today is eleventh session of the month. Equities typically rallied starting now in December. Since 1994, the S&P 500 has fallen in the month of December five times (1996, 2002, 2005, 2007, and 2014). Since 1994, seasonally, December is one of the best months for S&P 500 in terms of number of times since 1994 that it posted a monthly gain. April matches it for number of monthly increases since 1994.  

In the three years of falling S&P 500 shares since 2003 (2005, 2007, and 2014), the average losses is less than 0.5%. The average gain was near 2.5%, helped by a 5.1% rise in 2003, and 6.5% in 2010.