If you really want to go, go! People do manage it and if you are a sling-wearing type mum whose baby will sleep anywhere and can be reasonably relied upon not to cover every spare item of clothing with vomit/poo etc. etc., then it's perfectly possible.
But I think it can be tough on a baby of that age, especially if they've settled into a comfortable routine etc. Many babies find it deeply unsettling to lose the daily landmarks and familiarities that they partially use to feel secure. You will know your baby best, however, so will be able to judge whether the LO can handle it or not. It's a bit early to tell at this stage if your baby will be of an easygoing, anything goes type or not.
I've taken DD to a couple of festivals (not Glasto, but I know it well) and I would advise this:
-pick a festival that is reasonably family-friendly and where you won't have nightmares trying to find a half-decent loo to use etc. and won't have too far to walk everywhere (so Glasto possibly not the best option...)
-if you're not breastfeeding, consider the problems relating to making up bottles etc.
-it's handy to be able to stay off-site if you can, but think about the amount of stuff you will have to lug if you're camping for you and a baby and how feasible it is. A campervan might be a better idea.
-make sure you have some baby ear defenders as infant hearing is very delicate. Our DD slept brilliantly in hers!!
-slings are much easier to get around a rural festival site with a baby than a pram or buggy, unless you own a serious off-roader. We just made "nests" on a blanket for her when she wanted to sleep - or stood and rocked with her in the sling.
FWIW, DD loved the WOMAD festival and she napped brilliantly etc. while we were there, so enjoyed it with very fuss. BUT we didn't stay on site, arrived at lunchtime each day and left by 21.00 each evening. We did feel we had a good festival though, despite missing the late night stuff.
We hadn't quite started weaning her at the time, so didn't have to worry about food. But if you're BLW, then yes, you need to think about it as much of the food available at the Glasto catering stalls will have far too much salt/sugar etc. in them to be suitable.
Camping festivals are probably the place to think about using disposable nappies (if you're usually a washables type - we are) and handy babyfood pouches. I doubt it would muck up BLW too much if it was only for a few days.
HTH and good luck with whatever you decide!