The following chart plots the average performance of the FTSE 100 Index during August since 1984 (more info on this type of chart).
As can be seen, on average the market tends to trade flat for the first couple of weeks and then to rise gently in the second half of the month.
July 2013
The average chart for June was posted a month ago here. The following reproduces that chart and overlays the actual FTSE 100 for July 2013.