Most will be there from 08:30 to 16:00 when the event closes. The real start is 10:00.
It is in the whole 'Old City' part around Corn Streen, Small Street, and St Nicholas Street.
Avoid the multi story off Baldwin street its over priced...try Cabot Circus and some retail therapy if the other half is joing you parking there is just off the bottom of the M32.