This annual event happens near the start of September each year (the Monday and Tuesday after the first Sunday after 1st September) and supposedly exists to allow St. John's College to retain the private status of St. Giles' by closing it to traffic once a year (if anyone can confirm or deny this claim, please do!)
The fair occupies the entire road between [St. Giles' Church]? and the Martyrs' Memorial and usually manages to dominate the Oxford skyline with its 'Big Ben' rides and the local atmosphere with its noxious burger fumes. And the dodgems are too expensive.
On the other hand, completely redeemed by being one of the few/only places/times in Oxford it's possible to buy a Caribbean roti and cut sugar-cane pieces. This may be less relevant if you're not desperately trying to reclaim a tropical upbringing, and are at all squeamish about large chunks of unidentifiable and most likely toxic meat in strong curry.