Sorry, I couldn't find pricing data for that day. The closing price for the S&P 500 (GSPC) on December 31, 1984 was $167.24. It was up 0.6% for the day. The latest price is $6,913.98.
DATE
OPEN
HIGH
LOW
CLOSE
VOLUME
December 31 1984
$166.26
$167.34
$166.06
$167.24
80,260,000
Daily pricing data for the S&P 500 dates back to 12/30/1927, and may be incomplete.