I am trying to see the schedule on the Rome 2 Rio site for the RER C from Arpajon to Rue Palatine in Paris (for St. Sulpice) on May 28. I keep getting May 27 and May 29, but no May 28 (Sunday). There is a May 21 (Sunday) schedule, beginning with the current time in France for May 21, so I assume the trains do run on Sundays. Are there no RER C's running on Sunday the 28th? Can anyone help me find the schedule for that day for the RER C line? Thank you in advance for any advice. (I am hoping to attend the 10:00 a.m. organ concert and the 11:00 mass on that day, then visit the Cluny museum and return to Arpajon that evening, after dinner.)
According to the transilien website there is bus replacement (a 49 minute journey) between Arpajon and Juvisy with a wait before boarding your RER train there.
It also gives you a much faster bus/metro alternative
Rome2rio should only be used for initial research on transport info and it's a good place to start. But once you've got initial info you should be using the local transport websites, which will be up to date and have the best info for transport planning. The transilien.com website gives you info about this journey and mentions info about the replacement buses between Arpajon and Juvisy where you take RER C to continue your journey to Saint-Michel Nôtre Dame and then switch to take métro line 4 to Saint-Sulpice. You'd need to be on the 8:22 bus to make it in time for the 10AM mass. There is also also a bus from the stop Porte d'Etampes in Arpajon (bus DM151) to Porte d'Orléans in Paris where you take métro line 4 to Saint-Sulpice. As mentioned, all info is on transilien.com (and ratp.fr and https://www.iledefrance-mobilites.fr/en).
Merci, Mystique and isn31c. It looks like my journey will be a bit more complicated than I hoped, but will be worth it. In 5 previous visits to Paris I have missed the organ concert and mass at St. Sulpice. Thus time I am determined to get there!