It's possible to go to a concert or even a festival and not drink...but I think at 90 days I would have likely faced a lot of internal turmoil moments.
I think I would have not gone - even with Sabbath playing....
so...if you're going you need to be prepared for all that, and like others have said, have an escape plan (keeping in mind how difficult it is to 'get away' from things at a festival) - and above all remember -
that first drink is actually the one that does all the damage.
You have a while to prepare - I wish you well
D