Hi Catherine,
Fantasmic is my oldest son's favorite show at Disney World, and I consider it a must do as part of your vacation plans.
Previously Fantasmic was offered nightly, however the show was switched to a limited weekly schedule approximately a year ago. While there was never a formal announcement regarding the reason behind the switch, I would guess it was based on the amount of labor, time and money that goes into the performance. The show incorporates a good deal of cast, technology and labor and I can only guess Disney decided it was easier to meet all of these requirements on a more limited schedule.
Typically the show has been running two or three nights a week, however this is always subject to change, and additional shows are often added during peak travel times. Be sure to check the park schedule
here for the shows during your visit.
On a side note, if you are planning on enjoying the show, you may want to consider booking a Fantasmic dinner package by calling 407-939-3463. This package offers guests dinner at one of three locations (Hollywood and Vine, Mama Melrose and Brown Derby) along with entrance into a reserved area of seating for the show. Hope you get the opportunity to enjoy this amazing show!