Hi there! We are planning a June trip and I would be glad to share what I have learned thus far.

When I called to make advance dining reservations I was told they could not be made until 30 days prior to the day you wish to dine. The exception to this is the character breakfast at Confisco Grill at Islands of Adventure. I was able to make my reservations for this buffet breakfast. It is only held Thursday through Sunday and the characters include Spiderman, Thing 1, Thing 2 and Cat in the Hat. We are dining on June 12th and I had my choice of times starting at 9am. I think seatings are allowed until 11:30am but I'm not absolutely sure.
Depending on which park you wish to dine in advance reservations can be made at up to four places. At IOA you have the above mentioned breakfast at Confisco. I think they also serve lunch and dinner but without the characters. Mythos is also at IOA and serves lunch and dinner-we really enjoyed this place on our last trip.

. For IOA reservations call
407-224 4012 . At US Lombards and Finnegans allow reservations and you can make them by calling 407-224-3613 30 days prior to the day you wish to dine. We ate at both our last trip and enjoyed them.
I am not certain on the Citywalk restaurants. I don't believe you can make reservations with the exception of Emerils Orlando. I know if you are staying on site you can obtain faster seating and I did read somwhere that Bubba Gump Shrimp Co. at Citywalk does not honor the seating arrangement for Universal on site guests. Maybe someone more familiar can chime in as I am certainly not an Universal expert. This is just our second trip so I am still learning.

Hope this helps and Happy Planning!!