I want to go to CDG by RER from Chatelet this Saturday (24th)and arrive by 8:30 am. How much time should I allow for the trip? Can I buy RER tickets today for Saturday? If so, where is the most convenient place to buy them? My apt. is near the Hotel de Ville metro. Thanks kindly.

45 minutes. I cannot answer the others.

You can purchase an RER ticket in advance.

I THINK you can do this at any Metro station, using the automatic machine. (I don't know whether this can be done at a Metro ticket window.) There are two types of machines at Metro stations, one for recharging Navigo passes, the other dispensing tickets. The latter is the one you want. Some of the ticket dispensing machines now accept euro bills but some do not, so you should take enough change for the purchase, 9 euros. The machine controls are a roller bar to indicate choices, and a button to validate choices.

If this doesn't work, or if you'd rather deal with a ticket agent, to be certain, go the Chatelet station, or any station with an RER stop. Follow the signs to the RER. There will be an RER ticket window. You should do this during office hours as many RER ticket windows are closed in the evening.

Let us know how this works out.

Dave