Hint. Do not say anything at all during the "interview" about needing off the following Sunday. Remember, this is an interview, not a job offer. There is a good chance they will be interviewing others and will let you know something when they have made their decision. Now, if you should receive an offer, and they ask you when you will be available to start work, tell them the Monday or Tuesday (after the fest..Tuesday would be your best bet)considering you will be at an all day music festival on Sunday and may be too tired and not be your best on that Monday) Only you know the answer to that part. You don't necessarly need to give any details, just simply answer the "when will you be available question" using a date after that Sunday. If they do offer you the job duing the interview, and need you right away..well then, you can be honest at that point and explain your situation..and then wait to see what happens. Some companies understand those things, while others do not.
Well, that's my 2 cents worth! Good Luck to you!!