I booked in August 2008 for 3rd October 2009. We did originally want to get married on a Saturday in September but the venue we wanted was fully booked and only had week days left. So, if you like a popular venue then the sooner the better to avoid disappointment.
Also, the more you can done early the less you have to stress about in the final weeks 
Take your time, remember it's your day and you can do things as quickly or slowly as you like!