The weekend before London Open House Weekend, the rest of the UK does exactly the same thing, with a couple of thousand venues open to visit as part of Heritage Open Days.
Probably the world's most famous steam locomotive, the Flying Scotsman will be arriving at King's Cross station on Saturday.
Later this year, you will be able to watch the Freemen (and women) of the City of London exercise their ancient right to drive sheep across London Bridge.
A large formation of military aircraft, including the Red Arrows will fly over Central London on Saturday lunchtime, for the Queen's official birthday.
Fancy taking a hammer and smashing a prehistoric monument -- and doing so with permission?
After a couple of years of work, the Post Office Railway will start to take its first passenger trips through the tunnels under London next month.
Just once a year, a nature reserve in North-West London opens its gates to the general public, at just the right time to see an entire wood carpeted with bluebells.
For just one day, lots of venues will all join together to do lots of things, meaning that even with the fastest feet and most fortunate timing, you're bound to miss out on most them.
A large scale exhibition is currently on at the Guildhall in the City of London remembering the battlefields of WW1 in France, and the contribution made by the "doughboys" to the European war.
On the edges of London, within earshot of the M25 can be found a most marvelous museum, housed in an ancient barn.