The first public release of OpenGuides occurred on 29 April 2003:
So I think this calls for a celebration on or around 29 April 2013!
Given current patterns of activity, I suspect doing this in the UK
is the most sensible option. (I can offer accommodation for non-UK
I'm on holiday in Moreton-in-Marsh between 24 April and 1 May, so
if we're going to do this on the weekend before the anniversary
(i.e. 27-28 April) then Oxford would be the most plausible venue.
If we do it the weekend after (i.e. 4-5 May) then we could do it
either in Oxford or in London.