RER B ticket from Paris to CDG Airport

You can purchase a ticket any time you like. Cost is 10.30€, good either direction.

See this article: https://www.connexionfrance.com/article/Practical/Your-Questions/Travel-and-Transport/What-are-the-rules-for-validating-train-tickets-in-France
The gist is that tickets are good for two months, but you have to validate them shortly before getting on the train by time- and date-stamping them in a little yellow box in the station. So in other words, yes, you can buy the tickets the day before if you wish.

Tocard can correct me if I am wrong, but I believe you have to use the ticket to open the turnstile to get to the platform. Then, when you get to CDG, you have to use the ticket to open the turnstile to exit the station. That makes sure nobody buys a cheap Metro ticket to get on the train and then tries to ride it all the way to the airport.

Speaking of which, not all the RER B trains passing through St Michel/Notre Dame are going to the airport. Overhead monitors will let you know if the approaching train is the one you want.

Unlike métro tickets, RER tickets are sold point to point, with Paris collectively being a point. That is the reason the exit is controlled, you must have a ticket specifically for your intended exit point or CDG in this case. There are signs everywhere that métro tickets, tickets t+, are not valid on the RER outside Paris. Still, people attempt to use a ticket t+ on the RER B to reach CDG. I cannot remember the fine for riding the RER B without the proper ticket, but 10.30€ is not that much money.

I have never heard of a timeframe by which RER or tickets t+ must be used. They are good until used.

Good point that the northbound RER B platform is used by trains going to Roissy/CDG as well as trains going to Mitry/Claye. There are plenty of monitors displaying each train's itinerary, just be sure to check before boarding the RER B. Keep your ticket until exiting the transportation network.

Thanks everyone for the good advice

lnbsig, it seems that you are confusing main line tickets and Ile de France tickets (including RER).

An RER ticket never expires. The direction of travel indicated above does not matter. And it must be validated in a turnstil, and not in a yellow box.

Oops. Thanks, Chris.