Temporally speaking, it marks whatever comes next as the time the specific action mentioned would take place.
Ex.: "Tôi sẽ làm xong việc vào tối nay." = By tonight, the action of "I will get the work done" will take place = "I will get the work done tonight."
I tried "I want to say goodbye for tonight " which I think is a reasonably common way to say such a phrase in Australian