I missed the first one and I was wondering what the chances of another event will be.
No one knows for sure whether another Diancie event would happen, since nothing of the sort was announced. The last event for Diancie ended pretty recently, on 27 July, so you barely missed it. Chances are the next event for Diancie won't happen in a long time now, since one just finished.
Sorry, but I would suggest you just get one from trade now.
Hope I helped!