Because there are so many people at a festival, you only need a drop of rain to turn the ground to mud. Chelseas are fine if they are robust ones, or if the forecast is dry.
Also, it depends a lot on whether you are going to have a base - one spot where you can leave your stuff, because someone will be there at all times. If you are doing that, you can wear whatever you like and just take layers and wellies, as back up.
Don't forget mossie repellent, if you're wearing a dress.
Bum bag, worn at the front - fine. Worn at back or sides - risky because of pickpockets.
I disagree with a PP - a camping chair is much better than a blanket because, if it rains, you won't want to put the blanket down in the mud. Also pissed twats will trip over you more on a blanket.
Sorry if this sounds negative - it's not meant to be - I love a festival 😀