Hiber Day is a day of the year when the temperature is so low, it makes all the Amphibians freeze until the day is over. It affects cold-blooded creatures. It usually comes around at random times during winter. Sprig and Hop Pop Plantar normally have to warn the citizens of its arriving.
After all the snow melts, the amphibians are supposed to feel fresh and renewed after their awakening.
In "Snow Day"
Anne reveals to Wartwood Swamp that she is warm-blooded and that she doesn't freeze. After hearing that one amphibian goes missing every year, she offers to watch over them during the freeze. When she gets bored, she thaws out Sprig, although he is groggy and confused. Playing in the snow with Sprig, she gets distracted and doesn't notice frozen Polly was taken. When she notices, she and Sprig go searching for her. They find a cave, inhabited by a giant weasel. While Anne fights it off, Sprig goes to retrieve a hanging, frozen, Polly. They later discover the weasel is a mother and only takes the amphibians to feed her young. Feeling bad, Anne gives her omelet, and to her surprise, they like it and Sprig and Anne leave with Polly in tow.