We hope to train from Naples to FCO around noon on Monday, September 19th which, we understand, is a holiday (La Festa di San Gennaro). Is it important to purchase the ticket online as soon as possible and will the streets in Naples be crowded that morning? We assume we have to change trains at Rome Termini before we get the Leonardo Express (assuming no direct trains). Is that correct? Thanks in advance.
I would expect the area around the Duomo to be very crowded in the morning and within the Centro for the balance of the day. I wouldn’t necessarily feel the need to buy tix online as soon as possible except to get a good fare. But if you have a flight to catch and need a specific train, then it might be wise to get them soon. There is no direct train from Naples to FCO. You will have to change to the Leonardo Express once at Termini. I’d note that San Gennaro is the patron saint of Naples, so it’s a significant day for the city.