The closest one is Athens International Airport "Eleftherios Venizelos", which receives both domestic and international flights all year round.
Upon your arrival at the airport of Athens, you need to reach Piraeus port and embark on the ferry that transfers you to Aegina.
Ferries depart from Piraeus port daily, almost every hour. The trip lasts from 40 minutes to 1,10 hour, depending on whether your ferry is a regular or a high speed one.