I'm planning to go to Mykonos (Greece) in September, I've read that it is "post-season", are there still any parties or it is often empty/more familiar on those days?
September is not exactly 'post-season'. It's surely not gonna be like July and August but there are gonna be some parties till mid September and the island is not gonna be empty.
Here's a blog where they write about events in mykonos.
The nightlife of Mykonos is intense from June to September! But if you rent a motorbike to Mykonos or some other means, do not drink and drive!