This is a very, very simple job. Just go to some website like Google Finance or Yahoo Finance, and research the following: from 1950 - 2010, what was the increase (percentage wise) for the Dow Jones Industrial Average for the month of December.
When you're finished this task, please email me a spreadsheet that looks like the following
December 1950: +3%
December 1951: +2.5%
December 1952: - 1.4%
......... all the way to December of 2010.