I wouldnt of thought you could force roses to bloom for when you want them out on bloom.
But saying that it depends on what sort of Roses you've got.
The only thing is once the bloom has died of , dead head them and hopefully more buds will foem and you will get more blooms

Unless you have got roses that naturally bloom in late August you have a challenge. The big show gardens at Chelsea and other RHS shows are put together by professionals. They will used heated green houses, cold stores and other methods to get plants, shrubs and even trees to flower out of their normal season. Not something you can replicate in your average garden. You could check your local garden centres nearer the time to see if they have any flowering roses and place around in containers.