I assume that the Arc is at Chantilly for the next year or two. Have been to the Prix du Jockey Club so can imagine the fun and games walking through the woods from the railway station to the track. I can also imagine the 'attempts' to catch the train from the Gare du Nord to Chantilly and the drunken Brits returning to the centre of Paris. For information there is a great bistro/brasserie close to the station. Just about talking myself into going!!!!
For the British racegoer, Chantilly is a very impractical venue for the Arc meeting. Just looking at the ‘cheap’ way of doing a meeting in France (i.e. not travelling the day before and footing the bill for hotels and the same afterwards), there is no really viable option. To get the quickest transport to Chantilly from central Paris means getting the train that takes about 25 minutes (cost €8), from Gare Du Nord (good for Eurostar customers) to Chantilly-Gouvieux. To fly to Charles De Gaulle would need a bigger lead time for transit to central Paris. The earliest train that could be combined with the first Eurostar arrives in Chantilly just after 1pm. To do the whole two-day Arc weekend would presumably mean staying somewhere near Chantilly, where I would expect the hotel prices to rocket substantially now that the French have finally grasped the principles of supply and demand – it used to be possible to get a hotel near Longchamp for £50 on Arc weekend whilst last year it was nearer £90. There cannot be that many hotels in Chantilly as suburban Paris, so it will be pricey. I guess the best way to do it for just Arc Day would be travel over on Saturday, stay in Paris overnight and get an early train to Chantilly on Sunday morning. The two possible return trains do not get to Paris until after 8pm and will be packed. I think the free bus services will not exist as it is too impractical.
Agree with everything QM says above. It looks like a minimum two nighter with expense minimised by booking travel of whatever mode as early as possible. Is Eurostar booking limited to 92 days before date of travel? As I am 'out of residence' I am not sure but I think Easyjet timetables for October usually released about March. You can pack two people in one of the many Ibis/Ibis Styles hotels for a reasonable price. Hotels are clean and functional with free WiFi access. God knows how they will deal with the temporary 'stands' because the difference in the crowd who attend the Arc at Longchamp and the Prix de Diane at Chantilly must run into the tens of thousands. Still think I may give my usual trip to Auteuil in November a miss and give this 'novelty' a go!