Athens is an ideal year-round city destination with comfortable and favourable climate conditions for travel and sightseeing. Rainfall is minimal (estimated at just 40 cm or 16 inches per year) and Helios, the sun god in Greek mythology, smiles upon its streets over 300 days a year.
Late autumn is one of the best periods of the year to visit Athens. The morning and evening temperature is usually excellent but during the day, Athens can become very warm even in October and the beginning of November. You may get a day or two of showers and rain but usually the weather stays warm. The October weather in Athens is perfect for hanging out in cafes, enjoying dinners in outdoor restaurants and drinks sitting on a beautiful terrace in Plaka, Monastiraki, and Psiri.
There is still time to visit the local beaches or even to go to the islands where you will probably need a warm sweater for the evening breeze.

You may find more detailed information and forecasts at:
http://www.hnms.gr/hnms/english/