Hi Charles and welcome to the Disney Parks Moms Panel.
According to the Walt Disney World website, the
Symphony in the Stars: A Galactic Spectacular Dessert Party at Disney's Hollywood Studios is offered on multiple nights from January 5, 2016 through March 19, 2016.
Although the Check Availability calendar appears to allow you to search for availability for dates up to and including April 30, 2016, when searching, there are currently no tables available to book on any dates after March 12, 2016.
I haven't seen or heard anything to suggest that this Dessert Party will be extended for dates after March 19, 2016 but it might however be worth calling to check this with a Cast Member on (407) WDW-DINE or (407) 939-3463.
Wishing you a magical trip filled with memories which will last a lifetime!
Emma