March 13th at 2:45pm ET. Not a minute before, or after. If you ask before that people will think you're too eager but if you wait longer than that people will all be linked up and you'll be stuck living alone.
But to be serious, every city is different. So any advice here needs to be taken with a grain of salt. Go on your accepted student Facebook page and ask if people are looking for a roommate and reach out to people from the city you are in about the rental situation there