I think, as Malcolm points out with the road closure post (http://www.ttwebsite.com/forums/showthre...?tid=28458) the roads can be closed any time during that order. I presume this is applied for well in advance & has to be agreed by all.
Also, in that order details it does say "...on one Sunday only...." during either races. Don't know why that would be?
Also, it says "....Festival of Motorcycling, previously known as the Manx Grand Prix....", so there we have it, official word.