Is this a good time to go there? Whats the temperature? do u need jaCKET OR IS A hood okey to wear
Athens is a great city whatever the weather, but if you're expecting warm sunshine, choose another time. You could be lucky, but December is winter: it can be cold, and it can be wet (December is the rainiest month of the year).
Definitely a jacket - warm and waterproof.
It also depends on how tolerant you are. I used to live near Barons Court tube station and was constantly seeing people with just a hoodie on, waiting for the train sometimes in negative temperatures.