Just Googled 'Next Sales 2007' and the 'Next' means there are hundereds of links nothing to do with the Next stores though. Although I did notice that there was mention of the Next sale starting on 23rd July in 2004, so it could be sometime around then, sounds about right doesn't it!
the next sale always used to be the first weekend of when the kids break up for the 6 week summer hols. we always went away that friday night so i never got a look in, so if this is still the case that should be around 21st/22nd july??
A friend of mine that works for Next said that in order to prep for the sale and get the mark downs ready they write on all the tickets of the items that are going to be marked down in pencil the new price. This usually starts a week or so before the sale. Its written as a long string of numbers i.e. 000069900 is �6.99. So if you spot any writing on the tags...you know it can't be far away.
I work at a Next Clearance store in Essex and the sale is on 52 weeks a year. Sometimes the prices are incredibly low (�9 for suit jackets at the moment). Try and find one near you, it really is worth the trip if you dont mind last seasons stuff!