While there is no way to know for certain when
Extra Magic Hours will occur in each park until the schedule has been posted, there are certain trends that tend to be followed. For example, it's typical for Magic Kingdom to have extra magic hours on Thursday mornings and Sunday evenings. During peak times -- the weeks surrounding Easter -- there may be additional extra magic hours to accommodate the increase in guests.
If I were making reservations at a restaurant in the Magic Kingdom I would probably avoid Sundays & Thursdays, since that is the current trend. However, I would recommend checking the calendar periodically. Once the official dates are confirmed, you can always try making alternate reservations if there is a conflict.
Are you a resort guest? We sometimes LIKE making late breakfast or lunch reservations in a park that has extra magic hours. We can take advantage of the early opening, have a nice meal when as the crowds start to get heavier and go to our resort to rest or hop off to another park.