this information is different for each university - they do not operate set terms at exactly the same time like your schools generally do... though every university will have a webpage and this information will be available there.
As undercovers says, they all vary so you'll need to check the particular Uni you are interested in. I have found them to usually be the last two weeks in September, or later ones start first week in October.
Much university owned accommodation is rented out to non-students over the summer to generate additional income (and hence not available) but not all. If the reason you're asking is because you will be homeless or between lets at an inconvenient time before your course starts it is worth contacting the accommodation office of your university and asking their advice. They may be able to assist you.