From The National Trust website:

With English Heritage’s, the National Trust’s and the HHA’s major heritage sites participating this year, the total number of properties opening their doors has risen to a record 3,500.

Properties ranging from castles to factories, town halls to tithe barns, parish churches to Buddhist temples will be taking part, and this is a unique opportunity to learn more about local heritage and share your experiences with friends and family. A wide range of tours, events and activities also take place, bringing local history and culture to life.

More information on the program