Actually, to put an extra fine point on what the above posters are saying...something I have done quite a few times and maybe it's not something one wants to broadcast exactly, since it is skirting the Disney rules, but really how ridiculous is it to charge people for cancelling dining reservations anyway when like almost instantly another party scoops them up, but I digress, what I want to say is that I have decided I wanted to cancel some of my dining reservations here and there over my last few vacations over the last few years, simply because I changed my mind about where I wanted to eat and wanted to eat somewhere else, and I did this with less than 24 hours before the meal.
So, anyway, all you have to do is go into the app, to your reservation and modify it to another day, any day in the future...that's right, that is the loophole that Disney doesn't realize you can do, you can modify your dining reservation with less than 24 hours remaining to a day in the future, not just to a later time in your dining day, and after you've done that...guess what you now can do to that reservation? You can now cancel it without penalty because it is no longer less than 24 hours away.
I mean, some of Disney's rules really are cash grabs, nothing more, and when you consider the fact that as we are always reminded by Disney, these are not even really true "dining reservations" anyway - there is no table Disney is holding and saving just for your party that it would hurt them if you cancelled on short notice. As well all know, your dining reservation is not even activated until you check in at the podium and then you have to wait until there is an available table for your party. It's not like Disney is losing business or a restaurant could be crippled by guests cancelling at the last minute - Disney is the one place where there is no shortage of paying customers.
So, I don't feel bad about doing this once in awhile.
