It is likely to be warm and sunny during the day, and will get cool as soon as the sun goes down. IT WILL RAIN! Keep this in mind and you will remain comfortable throughout Festival. Bring sunscreen, make sure you drink plenty of (non-alcoholic) fluids, bring plenty of warm clothing, and some way of protecting your gear from the rain. Bring plastic bags or bins in which you can store your clothing and bedding if the rain gets really bad. Wool coats, cloaks and hoods are your best bet for keeping yourself warm and dry – wool sheds water well, and will still keep you warm if it gets a bit damp; synthetics will not.
One thing that you will not have to contend with is dust. The site is very green and grassy. Mud is more likely to be an issue – you should aim to bring some sturdy weatherproof shoes, boots or patens.