I was planning on going on Saturday specifically for the light show. Is this a good choice?
There is so much to see and do at Versailles, depends on how much time you want to spend. Did you want to tour the inside of the Château? The Trianons? Marie Antoinette’s Farm? Rent a bike?
The September 25 Saturday Night fountains light show will be really different. It will feature taped electro music throughout the gardens by some of France’s well known electro musicians, along with the lighting effects and 15 minutes of fireworks at the end (11:05 pm). Not your everyday Musical Gardens experience.
Getting back to Paris after the light show...You should be able to walk back to the Versailles Château Rive Gauche station in time to catch the last RER C train at 11:55 pm. Because of construction on this line, you will need to transfer along the way. The Citymapper app and Transilien.com site can help. I’ve attended a number of evening performances at the Château and have always managed to get the last RER C.
If you do miss it, there’s the Bus 171 to Pont de Sèvres where you can transfer to a night Bus to Cluny/St Michel area.
Overall details on travel to Château de Versailles:
https://en.chateauversailles.fr/plan-your-visit/practical-information