I think your best bet is to sign up to the mean fiddler mailing list, cos it will tell you straight away when they're on sale, and will give you a link for where to buy them
I don't think they will go on sale at a specific time. I doubt a lot of places will sell the the day the line-up is announced anyway. I dunno about internet ticket sites, I always buy mine in HMV, but even so, different websites will sell them at different times I suppose. In a way this is good, cuz that way all the touts can't buy them all as soon as they "officially" go on sale, like what happened with Glastonbury.