I'm pretty sure its whenever you want to take it. Though I heard that there's a down time. There's a specific month where no one can take the test. Can someone put more insight into this?
If you are planning on taking the exam in May, June, July. Those are obviously the busiest months since everyone is taking it then and applying. You should register for a day a bit in advance (maybe 1-2 months), or else everything might be full for the entire month when you want to take it.