I don't think anyone really knows, but ForumAdmin commented on it to the Mods at the time.
Looking back, the most likely possibility is that this news story broke that day, about parents cheating to get school places: http://news.bbc.co.uk/1/hi/education/8334503.stm
That triggered a reasonably lengthy thread on here, and may have resulted in a lot more people surfing "school places", "11+", "grammar schools", etc. It's possible that, as a result of the story, this site got a mention on a very big website somewhere that day, and that created a surge in traffic.
The numbers for Kent this year were massive though - I was watching them during the day and praying the server would cope!
Edit: Oddly I have just noticed that the most users ever online was at 2.38pm, and the BBC story was posted or updated at 2.36pm, so I am guessing that there was also a lot of Search Bot activity around that time.