Fantasmic: 30 days beforehand. I've heard it explained that you call on the same date you want, one month before: Want June 23rd? Call May 23rd. I've also read that you MUST call at exactly 8 a.m. 30 days before, or they'll be sold out. However, I made a mistake and called 27 days before, and they still had seats available for both the 9 and 10:30 Fantasmic showings, during Spring Break, one of the busiest times.
Restaurant PS's, up 60 days in advance. The earlier you call, the better your choice of seating times... I decided to switch to Blue Bayou 10 days out, and was only able to get a 4:30 or 9:15 dinner reservation. (the 4:30 worked out great, as it happens!)