"Ession" originated in Sydney and has many many meanings. To "have an ession" means to have fun, in no way in particular. You can have an ession anywhere, any time. To go to an ession usually means to go to a piss-up or a party.
1) Dewy: Dude c'mon we've gotta go to class.
Jake: Fuck that dude, let's have an ession.
Dewy: We can have an ession on the way.
Jake: An ession on the way to class?
2) Liam: What are you doing tonight?
Shezza: Havin' an ession. Turn up.
Liam: Why not.