Well, Wishes is a must-do for us on every trip. (Along with Celebrate the Magic). Because of that, I would plan to go the first night and then if it doesn't work out for some reason, I'd have a back-up day.
Monday -- BUT if you're at the park anyway on Saturday I would try sticking around because who knows what will happen on Monday weather wise, feeling tired, etc.
Personally, I would shoot for both nights. I love Wishes - it is my all time favorite nighttime show. We always plan for it at least one night while we are there - like others have said primarily because of weather. Saturday will be much busier, but I think it's doable.
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