The following chart plots the average performance of the FTSE 100 Index during May since 1984 (more info on this type of chart).
The chart shows that on average the market trades fairly flat for the first two weeks of the month, the third week sees a sell off, with final bounce back in the last few days.
The average chart for April was posted a month ago here. The following reproduces that chart and overlays the actual FTSE 100 for April 2013.
In an average April the market climbs steadily throughout the month. This year, the market started weak, then recovered during the latter half of the month.