Sorry, I couldn't find pricing data for that day. The closing price for the S&P 500 (GSPC) on December 28, 2018 was $2,485.74. It was down 0.5% for the day. The latest price is $6,552.51.
DATE
OPEN
HIGH
LOW
CLOSE
VOLUME
December 28 2018
$2,498.77
$2,520.27
$2,472.89
$2,485.74
3,728,440,000
Daily pricing data for the S&P 500 dates back to 12/30/1927, and may be incomplete.