This page is still showing this year's concerts, but they may update it for 2019 soon. Make sure you read the descriptions because it's showing events for Six Flags across the U.S. https://www.sixflags.com/Event-Category/Concerts?page=1
Since the 2018 season is still open I would check back the end of January. They start pushing season passes come mid February.
I would assume here as they get booked or by email.
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