I emailed Mill City about the tickets being onsale online for the event, and got this reply:
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Dear Maria,

Thank you for yoiur interest in Mill City. We are very
excited about The Artist playing and it should be a great
weekend. You will be able to purchase tickets online later
today or at the latest tomorrow on our secure server.
Please check back a little later.

Regards,

Doug Hamley
Mill City Music Festival
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To Anonymous, I apologize for saying that you had gotten the info wrong. It's just when I saw the website this morning I didn't see anything about tickets being onsale online.