Any day as long as you have a season pass. They only give a certain amount of tickets for friends PER SEASON PASS
As far as I know for October you can bring a friend free on Fridays when you have a season pass
If you download the six flags app and select your park it will tell you exactly when you can bring a friend free
The days change month to month, you can Google them regularly
Thats only available if you have a season pass.
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