Transit strike

here's the issue: as of now, there's no transit strike planned for August.

However, the central portion of the RER C -- that is, the part that runs along the river in Paris -- is closed this month for repairs.

From the RATP website:
https://www.ratp.fr/travaux-manifestations

Travaux de maintenance
Lieux Entre Gare d'Austerlitz et Javel / Avenue Henri-Martin
Dates Du dimanche 15 juillet au samedi 25 Août toute la journée
rer En raison de travaux de maintenance, le trafic est interrompu entre la Gare d'Austerlitz et Javel / Avenue Henri-Martin, du dimanche 15 juillet au samedi 25 Août, toute la journée.

Between Gare d'Austerlitz and Javel/Avenue Henri-Martin
From Sunday July 15 through Saturday August 25
Due to maintenance work, there is no traffic between the Gare d'Austerlitz (RER C station) and the Javel/Avenue Henri-Martin (RER C stations).

So you'll have to find another way to get to Versailles.

For example, you could take the Tramway 3B to to Pont du Garigliano and catch the RER C there. (You can catch the Tramway 3B at several points, example at Balard at the end of line 8, or Porte de Versailles on the line 12, Porte de Vanves on the line 13, Porte d'Orléans on the line 4, etc etc )

Thank you, Kim. I was going to start out at the Invalides station. I wish I understood the Parisien transit system to immediately understand the routes you suggested. I will have to take a closer look.

From Invalides, you could take Metro Line 13 to Duroc and change there for Line 10 and take it to Javel Andre Citroen. At JAC, exit the station and walk 2 minutes to the Javel stop on the RER C Line where you can catch the train to the palace. If you buy a ticket to Versailles from Invalides, it will work for you to access the Javel station and to exit the RER station in Versailles.

Thank you, JHK. We pre-bought the tickets for Aug. 25. We should have bought them for a few days later. We would have avoided the partial closure.

Luckily you've thought about this ahead of time and thus have time to figure it out. It will be a little bit more of a pain to get there, but won't be the stress of heading out from your hotel to Invalides only to find there's no train running from there and have to figure it out on the spot!! You're thinking ahead, so you can adapt more easily and with far less loss of time. Smart!!

JHK's suggestion will work well.