If you are willing to fight the crowds, going on the 4th of July is the day to go. In past years, there have been additional fireworks and a patriot spin to the day's activities including a swearing in of 1,000 citizens last July.
However, the Magic Kingdom doesn't really have a table service restaurant with views of the fireworks. The Crystal Palace and Plaza Restaurant could have a few seats with views as well as limited views at Tony's Town Square Restaurant (need to sit outside), but they may not be the best. You may have better luck by eating at a counter service restaurant like Casey's Corner, Cosmic Ray's Starlight Cafe, and Tomorrowland Terrace Noodle Station (open seasonally) which offer outdoor seating with views of the fireworks. However, you will need to get there EARLY. Many guests will begin saving spots early in the evening.
Might I throw out an additional idea? If you'd like to avoid the crowds and the need to reserve a spot early, you might want to consider celebrating at one of the MK resorts. 'Ohana (more family friendly), California Grill (with an outdoor balcony), and Narcoossee's are lovely restaurants that have views of the fireworks. A favorite of my family is to eat early and then watch the fireworks from the Polynesian beach; the kids can run around, play in the sand, and enjoy the Electrical Pageant Parade afterwards. It is isn't quite the same as being in the MK, but it can be just as magical with less stress.