One of my favourite annual events, the London Open House Weekend, has confirmed the dates for 2009 – and I am now marking the weekend of 19th-20th September as unavailable for anything else.
I usually buy the Event Programme as it does contain a lot of information and acts as a useful guide for the rest of the year of “places that might be interesting to visit” if I can’t make it on the open weekend and they are open on other days.
Usefully, they are taking pre-orders for the brochure and orders placed before 30th April will be sold at last years price of £4.50 (I presume there will be a price rise later?)
Go here to pre-order the guide.

I usually pick up a copy for free from a library. Not sure if you get it earlier if you order, though.
Not sure – but I am incredibly lazy and prefer things to just appear suddenly in the mail box without any effort on my part ;)