Doors Open Day for any place that wants to take part.
It's a great opportunity to get into places that you'd never otherwise find yourself in a postion too, only it can mean a lot of reading of lists of participants to find the ones you want, and it has to be said they're usually well buried in long lists of churches and similar.
Nothing against churches, but they do swell the lists, and always puzzle me, as most can be got into without needing a special day, unless they're closed for restoration, or just closed.
I remember the old days, when the list fitted on a fold-out piece of paper. Now, forests would be in danger if it wasn't for the web sites