The following chart plots the average performance of the FTSE 100 Index during December since 1984 (more info on this type of chart).
Historically, December has been the strongest month in the year for the market. As can be seen in the chart, the market climbs steadily throughout the month, accelerating slightly from the middle of the month.
US election years
The following chart shows the average performance of the market in December in US presidential election years.
Unlike November, there is no great difference between the average market in December for US election years and all other years.