The exam is currently in Beta. That is, Sun are about to test their questions on a number of candidates who have already signed up for the beta exam. It is no longer possible to sign up for the beta exam.
When enough candidates have completed their examination, Sun will determine a pass mark based on the overall scores of all candidates, taking into consideration comments made by the candidates.
Shortly after this, Sun will make changes to the questions based on the feedback. They will then convert this exam from beta status to a standard examination. The standard examination will have far fewer questions than the beta exam (the entire set of "approved" questions will be added to a pool, and each candidate from then will get a unique subset of those questions).
From this time onwards (possibly somewhere around 2 - 3 months from now) you will be able to sign up for the SCJA exam anytime you like. There will be no special dates - it is entirely up to you.