The amusement park's original Barbershop Quartet "Four of Kind" helped kick off the festivities Friday.
The celebration will continue all day and all night with a special fireworks display.
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Guests are invited to step back in time and explore the evolution of the park.
And for the first time in nearly 10 years, the iconic American Eagle wooden coaster is back with side-by-side racing.
"American Eagle has been a classic of Six Flags Great America for more than four decades," said John Krajnak, park president of Six Flags Great America. "Bringing back racing is a meaningful way to celebrate the nostalgia guests remember while delivering the kind of thrilling, shared experience that defines our park today."