The following chart plots the average performance of the FTSE 100 Index during January since 1984.
As can be seen, historically the market tends to rise for the first two or three days in January and then sells off quite strongly over the following two weeks. The second week of January is the weakest week for the market in the whole year. Then, around the middle of the third week, the market has tended to rebound sharply.
Other articles about the market in January.