Hi MJ!
I highly recommend making Advanced Dining Reservations (ADRs) as far out in advance as possible. They can be made up to 180 days in advance
here. There are some amazing restaurants at Walt Disney World and they are very popular; many book up close to the 180 day window, especially for the busier times of the year.
Le Cellier Steakhouse in the Canada Pavilion at Epcot is particularly hard to get a reservation for.
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.
Have a magical vacation!