According to Galleria Borbonica tours of the Bourbon Tunnel can be done on "Fridays, Saturdays, Sundays and on holidays". My question is... will September 19 be a holiday that they'll be open? According to the Bank Holiidays in Naples September 19 is a holiday in Naples (Feast of St Gennaro).
The Bourbon Tunnel has three types of tours:
The latter two require reservations whereas the first one does not. But the URL to book tours in English doesn't work. The "Reserve your tour" button on the Adventure Tour takes you to Adventure Reservation gives an "error 404".
The Italian language link, however, does work: Prenota la tua visita.
But it only goes up to September 3!