I had the very same question. I started planning our May trip back in January. What I did was look at the park hours from last year at the same time we are going. There are no holidays during our dates. This gave me an idea about probable hours. When the magic 90 day PS time came around, I went on and made the PS's that I thought we would need. Then at two full months out (end of Feb for our May trip), the real park hours were released, and I just made adjustments based on anything weird in the true park hours. For example, Magic Kingdom will be open until 10 on one night while we will be there. Disney has scheduled Wishes and Spectro for that night. We really want to see Spectro, and the only other time it is scheduled while we will be there is on a Saturday night. All the advice I was seeing said to schedule Spectro on any night but Saturday if you have a choice. So, I adjusted our plans to see Spectro and Wishes accordingly.
I've been reading these boards, reading the Unofficial Guide and also using TourguideMike.
If you have a general idea about what you want to do, you can make adjustments as you go. Hope all this helps you.
It's seems like we just answered a very similar question about September's hours. You can use last year's operating hours as a guide. Here is the link: Sept. Hours
But, there is a difference this year than last, because WDW has committed to showing Wishes every night at the MK, except for hard ticket event nights. So, September will be a month that benefits from this policy, because the MK will have to stay open until at least 8 pm for it to be dark enough to have the fireworks. Be aware that Night of Joy will take place on Sept. 9, 10, and 11th, so the MK will close early those nights, and there will be no fireworks at park closing.
Have a great time planning!