To get "Airside," you have to pass through the country's Immigration exit check. To get to the Immigration exit check, you must first show your boarding pass and go through security. To get a boarding pass, you must check in to the flight, and be issued a boarding pass by the airline or print it out before you get to the airport. (Some airports switch this, and make one pass Immigration before showing your boarding pass and going through security.)
Both the boarding pass/security check and the Immigration exit check must be open for business and receptive of your entry when you approach them; not all these functions may be available 24/7. Boarding pass/security may refuse your entry if you present yourself the day before the flight's departure.
So the answer to your question depends upon:
being able to get a boarding pass for your flight the day before it departs; and
having the airport's security line and boarding pass inspection and exit passport control functions open and operating when you want to pass, and willing to process you long before flight time; and
having the airside terminal remain open during the time you wish to wait. You might, for example, pass into airside in the evening, hoping to stay overnight and take your flight the next day...but find the airport closes the terminal at midnight, and requires everyone to leave until the next morning.