The clearest days in Athens are wonderful, with sunny, clear views across the city and its monuments. It can be this way in any month, but at times Athens suffers from noticeable pollution. It's clearest after windy or rainy weather.
Winters during the mid-November to end of February period can be cold, with daytime temperatures occasionally falling to low single digit figures (centigrade), snow is rare. You can expect the norm to be 10-16 degrees. The likelihood of rain is higher, mostly in January and February.
Summers are hot and dry with 28-38 degrees the expected range. Consider these conditions to apply from mid-May until the end of September.
Spring and autumn tends to have a sample of both summer and winter weather, in temperature and rainfall, usually somewhere in between the two - but these periods are typically mild.
We should point out that, in common with worldwide weather changes, recent years have seen greater unpredictability in climate.