Here's the link to the schedule. The earliest you can book is May 13 for Brilliance and Radiance, if you're sailing on or after June 10/13 respectively. "Unfortunately" for us we are sailing in 43 days, so we'll have to do it on board.
I completely agree. We live in Barcelona and have been to Europe (Italy too) in August, and I can assure you that not only everything IS 10 times more crowded (or more) but also some services are not working 100% (buses, subway, etc). Besides, some places usually tend to increase prices in August (seems that being on holidays makes you richer :rolleyes: ). Therefore, since you have the chance, do it in September! :)