It will be on 19th September, Respect the Aged Day (a national holiday) and takes place as the 17th Sekai Isan Gekijou (World Heritage Theatre) event. According to it's official website, Sekai Isan Gekijou brings together Japan's history and heritage with popular artists for one-off concerts.
Tickets will be going on sale 7th August from 10am (Japanese time) but you can also buy them on the day.
This is a special one! According to the Tomioka Silk Mill page it will take an hour by Shinkansen from Tokyo to a station in Gunma, then more time on top to get to the silk mill. Still, it'd be worth it. The building looks beautiful!