This isn't likely to make Dick Enberg's justly celebrated Olympic moments, but it should. Of such reasonableness is a secure future made. That's this philhellenes' perspective. It gets cold and rainy in the winter, nothing to impress someone from Norway, but too cold for the beach. I've compared weather information from a number of sources, tried to resolve the contradictions that I found to give you a consensus opinion.
Greek temperatures vary more by altitude and exposure than by north to south.
There are only slight climatic differences from the islands to the far north. Height above sea level and exposure to ocean breezes make more difference. Both have good weather and fewer crowds. Most things are open full blast then. Winter is winter.