Hi Kathryn!
Thanks for your question. I have never eaten at
The Diamond Horseshoe at
Disney's Magic Kingdom, but I've heard it's enjoyable. Perhaps I should just go on a food crawl with me and my family at Magic Kingdom, hitting all the restaurants exclusively, right? I'll put that on my Bucket List.. :-)
This is all subject to change, but as best as I can tell you,
The Diamond Horseshoe is open for lunch at least most of the year, but still seasonally for dinner. For example, from what I can gather, it is open Spring 2020 for lunch and dinner, but Fall is lunch only.
My best suggestion is to make advanced dining reservations during your time of travel, pick the dates/times you're interested in, and see if there's any availability. You can only make reservations 180 days prior to your arrival, so bear this in mind if you're looking to travel, say, sometime in year 2020.
Thanks again, and have a Mickey Magical Day!
Terry