I disagree with all of my esteemed FT members..... the general advice given by almost every forum like TripAdvisor and Matt Barrett's ( other than FT evidently) is to be in Athens the night before an international flight. Especially when you are on two tickets. There are too many things that can go wrong coming from the islands the same day. (weather, wind, strikes, etc.). It comes down to your comfort and stress level. Having been Greece dozens of times over the last few decades....we always are in Athens the night before.