Will not be attending this year, sadly, but went the last two years.

A few words of advice:

1) Get your return train ticket in advance! If you stay for the last race on Arc day, or even the last G1 (IIRC, the arc will be 5th or 6th race of 9 or so, with 1-2 G1's after it), you're going to be hard pressed to buy a ticket and make the train. This will likely leave you waiting 2-2.5 hours to get the next train (or, I am told that some people just jumped on the train last year, [i]sans billet[/i] of course I cannot comment personally on that!). So, buy your return ticket in advance. Also, don't count on being able to get a taxi from Chantilly back to Paris, unless you've booked that in advance. We were able to find one Saturday, but not Sunday.

2) Chantilly is phenomenally beautiful. If you pictured Saratoga, but 500 years older, and French, you'd be starting to get the sense of it. I'd urge you to go for both days, Saturday is also a really great day of racing (5-6 G2's and G3's IIRC), and will allow you to figure out where there's a bathroom and a beer/wine stand close to your seat, and play with the betting terminals a bit, because on Arc day the English-speaking tellers see *huge* lineups. 2016 Arc at Chantilly, I lined up to bet the Arc at 25MTP, and only got half my tickets in (that was at a French language automated terminal, in other words, the shortest lineup on the grounds).

3) If you haven't bought tickets yet, remember that it's going to be fall in Paris, so it isn't impossible to see some rain. It drizzled a small amount both years I went. You'll have to decide whether covered seats are worth the spend for you.

Please let me know if you have any questions I'd be very happy to help however I could.