Random Dancer, some of this could be bad news...
Wherever I contradict advice already offered, you should take it that I am right because this information is based upon actually being there last year and plans made so far for this year.
You do not need to pre-book entry tickets for Arc Day, you just buy the one price ticket on the gate like the rest of us. Unless you want to be in a couple of areas that are reserved for the likes of Racing Abroad tour members or the social elite, you will have access to all areas (parade ring, grandstand, etc.) as there is no “members’ enclosure” or the like in Socialist France. I went both days last year so I was on a two-day ticket purchased on Saturday but I expect that the ticket for just Sunday will be around €12 to €15.
On Arc Day last year the first race was at 2:00pm and the last of the nine races was at 6:50pm. Usually the last race is a handicap and I would suggest that you will probably want to give it a miss and head for transport back towards the city centre after the eighth, especially as the free buses have substantial queues.
The racecards are no longer free on Arc weekend and cost €5 last year and run to over 130 pages – if your French is not up to much you will not be reading much of it but it does show you the colours and is a nice souvenir! Just inside the entrance, you can get a free B&W A3 sheet that has all the day’s races (runners and riders) listed on both sides.
Have you already bought any travel tickets? What you need to know is that in travel time terms Charles de Gaulle airport is a long way from Longchamp racecourse unless you are getting a hire car or a taxi direct (not recommended).
To get to Longchamp racecourse from Charles de Gaulle airport, you firstly need to take the RER train to central Paris. There is only one line from the airport and that is RERB. If you are not going back the same way, there is no money-saving idea that I can offer you so you will just have to pay the full single fare, which is about €10 to central Paris. At the airport, you should have no problem using English at the ticket office windows. Pick up a free map of the transport system at the airport as it will help you get on the right trains as they just have destinations on them not directions and the stations are signposted the same.
To get the free bus service at Porte Maillot, you need to get off the RERB at Châtelet Les Halles. You will need a new ticket for Metro (RATP) Ligne 1 westbound (destination La Défense) to Porte Maillot, which is quite slow as there are ten stops. What you should get is a single T+ ticket from one of the ticket machines as you are only travelling within Zone 1. Once you get above ground at Porte Maillot, you need to find the bus stop for the free buses to the course. The bus ‘station’ is quite expansive but look for the stop where there are lots of people!
If you are going out on the Eurostar, you need to be on the first one (as I am) that departs St Pancras at 08:19 and arrives at 11:47 (local time). It does not take over three hours but the French are one hour ahead of us. A return Eurostar needs to depart at 20:13 (local time) or later from Gare Du Nord.
When returning, assuming that you are using Eurostar, you need to get the free bus back to Porte Maillot. Then get Ligne 1 eastbound (destination Château de Vincennes) back to Châtelet Les Halles and then take RERB one stop to Gare Du Nord.
In terms of travel, what I have just explained is the easiest way that I can think of doing it for a newbie, rather than exactly what I am likely to do myself given that I am on the Eurostar both ways having bought a ticket months ago.
I recommend that you go and have a look on the
Paris By Train website as you can download PDF maps of the Metro and RER on there.