To be honest, I wouldn't bother. I never went, and my friends (really smart - honestly) who went came back and said that everything flew past them. I mean, it's like 20 weeks worth of material in 1-2 days?
If you really want to get advanced, I would just slowly start doing the coursework, and hire a tutor or something to help you with what you don't understand. That way you get way more out of the same price.
But don't stress yourself out during these holidays. Relax as well - take my advice; I studied a LOT during the summer holidays last year (because I was really behind on one subject - Latin) and regretted it because I didn't feel refreshed the whole year. The other holidays I decided to just totally rest and did so much better.
I know that it's easy to get afraid, especially if you feel you could have been more prepared, but please chill out as well
