From the itineraries I gathered through www.ratp.fr and Google Maps it's not possible to get to Paris (Denfert-Rochereau) from CDG (T2B) by train (RER B) on the weekend (2nd October). How can I get there?
There is a replacement bus service called the L2 line, blue color, between CDG T1 train station (leaving from the Roissypole bus loop, there should be plenty of signage) to Aulnay-sous-Bois RER station. Purchase your RER B ticket from the CDG T1 station as normal and board the L2 bus to Aulnay-sous-Bois, then use the RER ticket to continue your journey towards Paris on the RER B (direction Saint-Remy-les-Chevreuse / Robinson).
More info on the replacement bus service from CDG to Aulnay-sous-Bois.