Due to multiple factors, expect to potentially pay more when booking in January. For low rates, May can be a great choice for travel to Jerez de la Frontera. Flights to Jerez de la Frontera from Chicago O'Hare Intl Airport in May are about $694 on average during May, but can be found for as low as $28.
Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Chicago O'Hare Intl Airport to Jerez de la Frontera as you normally would.