10 am every day, but there is a thing called "Sweet Start" where the hollow, kissing tower hill, and founder's way are open at 9am.
10am unless you are season pass holder then it is 9am
Look into the Sweet Start. I believe this is it you are staying at one of the 3 Hershey properties.
The best deal is to buy your ticket ahead of time because you then can visit Hershey Park for free starting 2 1/2 hours before closing time on the evening before your full day and evening of fun! This is called Preview Visit, and if you are staying overnight, it is an excellent way of seeing everything. They mark it on your ticket. Have fun!
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