Hi!
You should make dining reservations for full service restaurants as soon as possible. You are allowed to make reservations up to 180 days before. In my opinion, during the busier times of the year, it's gotten to the point for a couple of restaurants that you actually have to make the reservation 180 days out. That's especially true if you want to eat at a particular time, you have a large party, or you are only a party of 1 or 2 guests.
Le Cellier Steakhouse in the Canada Pavilion at Epcot falls into this category.
However, Walt Disney World has a lot of restaurants and generally speaking you can make a reservation for some of them as late as the day before or sometimes even the same morning for a lunch or dinner reservation. I usually do not have a problem making a reservation at Disney's Hollywood Studios for Hollywood & Vine, Mama Melrose's, or the Hollywood Brown Derby just before going on a trip. Likewise at Disney's Animal Kingdom for the Rainforest Cafe (but not the Downtown Disney location), Yak and Yeti, or Tusker House. At Epcot, I can generally get a "last minute" reservation for the Biergarten in Germany, Tokyo Dining, Nine Dragons, or Restaurant Marrakesh. I've never had any luck in getting a reservation for a table service restaurant in the Magic Kingdom unless it is made well in advance!
It's also easier to get a "last minute" reservation at a resort hotel than in a theme park. However, some of the hotel restaurants are very popular and you should try to make reservations for them as soon as you can.
You can make reservations
on-line. Have a magical vacation!