Hersheypark is a popular destination all year round, especially during the holiday season. The amusement park knows that people plan trips to their park many months in advance and they want to make sure they can accomodate all the tourists coming to visit.
On their website they do a great job of showing every day what their specific hours will be. You can go directly there by visiting http://www.hersheypark.com/park-info/hours.php.