“Kitten season” is the time of year when the majority of kittens are born and, subsequently, the time when most shelters and rescues are inundated with underage kittens. In most areas of the United States, it runs from March through September. Keep in mind, however, that kittens can be born at any time during the year, and in warmer climates, “kitten season” can run longer or exist all year.