Northern Counties
No one knows for sure - Even those claiming to have inside information should be taken with a pinch of salt, as they could be out and out lying or have information skewed by being passed through several people before getting to them. Add to that the fact that the plans are as fluid as the world outside BA, and change on average about 18 times per week.
Looking at the previous 2 events, they seem to be opened once per year around the end of the year, but it's not a large sample size so there's no real trend to speak of.
In short, trying to predict when it will open is a mug's game, not even BA know yet. The best plan would be to ensure that you keep up to date with what's going on, work on your weak points constantly, and use the time now to build up an idea of what the selection process is like. That way you'll be in the best position to apply when it does open.