AADSAS takes time to process applications and does not send out the first applications until the 3rd Friday after opening. To be in the first batch, submit your application before the 3rd Friday after the application opens.
Early would probably be anything before August. But anything from August to September is still fine. Any time after that and you're considered late. Honestly, being first batch doesn't really affect your application. Lots of people make such a huge emphasis on "applying early"... but as long as you're not late, "being early" won't have too much of an effect on your application.