CGT Events are what trigger a capital gains tax liability. There are a lots of different situations that trigger capital gains tax, but by far the most common one is selling an asset.
When you sell an investment, you will either make a capital gain or a capital loss. Unless there is an capital gains tax exemption or a rollover provision, you will need to calculate your capital gain. You should seek advice from a professional tax accountant in order to determine when your CGT Event took place.
The date of sale is the date in which you undertake to sell the property, not when the sale is finalised. So for real estate, the CGT Event happens when you exchange, not when you settle. It is important to remember this when you are determining which year the capital gains tax event occurred in.