The 'joint session' is jointly held between the Mind Association and the Aristotelian Society, and is the largest annual gathering of philosophers in the UK. It has been held at nearly every major university in the UK and Ireland.
The Aristotelian Society has more information about the Joint Session including its history.
This Joint Session is special in two ways. It is the first to be held on weekdays. Second, it will result in the 100th issue of the Supplementary Volume which publishes the symposia of each joint session. Does that make it the 100th Joint Session? No -- there were a few before these volumes were published. But it is the 100th anniversary of Reading University!