We are season pass holders at Hershey and the first few weekends that they are open for the summer are a GREAT time to go. We typically go a lot in May and June, and then hardly at all in the second half of July and August since it's so hot and crowded. The least crowded times (most weekends in May will be 10 min waits for almost everything) are of course the first and last 2 hours they're open. If you are able to go on Saturday instead of Friday, you can take advantage of their preview program, where if you buy a ticket for Saturday, you can go for free for the last 2.5 hours Friday night. The Giant grocery stores sell discounted tickets I think. You can follow them on Facebook, they occasionally post ticket deals, but I think they are usually for multi day tickets.
A couple other things- there is an app that's pretty good and has ride wait times. If you get there when the park opens, head all the way to the back to Laff Track first- it's the newest ride and pretty fun, but it's slow loading and the lines get really long. You really shouldn't have a problem with any other waits in April/May. Have fun!