Zoo residents Ahava, a 5-month-old snow leopard cub; Whirl, an Amur tiger and the majestic bison all enjoy the colder temperatures.
Each Tuesday and Thursday in October and November are free with a reserved ticket.
Online reservations are required with times available in 20-minute increments from 9:30 a.m. and 3:30 p.m.
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Parking tickets also have to be secured before arriving at the zoo, and a parking fee of $15 per vehicle still applies, according to the zoo.
Only outdoor animal exhibits are accessible to guests as the Brookfield Zoo follows Phase 4 guidelines from the state, according to a release.
In December, the free days move to Mondays and Tuesdays.
"Dinos Everywhere!," which features 40 animatronic dinosaurs, is still open to the public through November 1.