Hi there, i've just moved this thread to an area where (hopefully!) more people should see it, and feel inclined to help you out with some info.
As far as i'm aware, there will be such things as:
An owner's club shop (provided/run by the Alfa Romeo Owner's Club)
Support from Piccadilly Motors, an Alfa dealer local to the area which will be bringing along some of its models
A catering marquee
A 'trade village' with various bits and bobs for sale
A concours competition
Etc etc. There will also be special parking for Giulia's, Alfetta's, 155's and GTA's, due to each having a significant anniversary.
Depending on the weather, you should expect a fairly large turnout of all manner of different Alfas.
Just to confirm, as already stated by Ali Tait, National Alfa day is to be held on the 12th of August at Newby Hall, Ripon, Yorkshire
Hope this helps