From the Goodwood Festival of Speed FAQ page:
Can I buy a ticket on the day?
Answer: Entry is by advance ticket only. It is not possible to buy ticket(s) on the day, and there will be no admission without a ticket.
So if you want to go, buy tickets now.
It’s been too long that I have been to this magnificent garden party for petrolheads. You won’t find another event so packed with many of the most important cars and car people. Like the Mille Miglia this is an event you should experience at least once.
2013 is the year the Festival of Speed celebrates its 20th anniversary. The poster shows an Alfetta Grand Prix racer from 1950/51, bursting out of the Goodwood House.