The warm period in December has a word in Norwegian: kakelinna, traditionally thought to be caused by excessive baking before Christmas. https://no.wikipedia.org/wiki/Kakelinna
https://en.wikipedia.org/wiki/Alcyone_and_Ceyx
The phrase "halcyon days" seems to be used in English also for the warm days in the middle of winter:
https://en.wiktionary.org/wiki/halcyon_days
I'm not sure how common this warm mid-winter period is in general but I've certainly noticed it in the 15 years I've lived in the UK and it must have been common in Greece in the past also. In recent years, also, but Greek winters in modern times tend to be very mild anyway. Nothing like in Norawy, I'm sure :)
We have our share of bizarre weather this winter - while it's customary for snow to melt right before Christmas - I can't recall there being this little snow not only in December - but through Christmas and well into January.
I just went for a walk and watched people skating on the nearest pond - which is usually frozen this time of year, but also covered in a meter or more of snow.
Meanwhile they're having a white Christmas in Bergen.