Is it necessary to buy a train ticket in advance to travel from Paris to Versailles? Can it just be purchased on the day you travel? If we buy a Paris Pass is it best not to plan to use it for the "All day " spent in Versailles?
Carolyn, Yes, you can easily buy a ticket to Versailles on the day of travel! The easiest travel method is to use the RER "C" from Paris to Versailles Rive-Gauche station. The trip is about 40-minutes each way. When you exit the station, take a right for a few blocks and then a left on Avenue de Paris (which turns into Place d'Armes). Happy travels!
I assume you are talking about the Paris Pass that covers entrance fees to various attractions plus a zone 1-3 Paris Visite transportation pass. IMO, it is not a good deal. And it won't cover your ride to Versailles which is in zone 4. A single ticket from anywhere in central Paris on the metro and RER C to Versailles-Rive Gauche only costs €3.05 each way. Be aware that the RER C line divides on the west side of Paris so be sure to get on a train that is going to Versailles-Rive Gauche. Trains run about every 15 minutes and your ticket is good on any train. For a list of all your Paris transportation ticket and pass choices with links to detailed explanations of each one, go here. Versailles and many other top museums and monuments are covered by the Paris Museum Pass.