Hi pjrotramel,
Welcome to the forum!
Which Portuguese route are you planning to walk? You've posted this in the "caminho interior" section, which is a caminho that goes from Viseu to Verin in Spain (on the Sanabres) via Chaves and then onto Santiago. But whichever Portuguese route you're thinking about walking, Dec-Jan can be very rainy in Portugal north of the Algarve. There was a forum member who walked with his wife and nephew in that time frame maybe two or three years ago -- I just can't find his posts or remember his name. Anyone else remember? His wife was Japanese, I believe, as was his nephew, and he had some great stories to tell. But lots and lots of rain as I remember.
I've lived two academic years in Lisbon. During one of those years, it never rained at all during the winter months, but the second year was like monsoon season. These were 2004-05 and 08-09, so it's essentially impossible to know what will await you! Temperatures will not be really cold anywhere in Portugal, though, except maybe up in the Serra d'Estrela.
Good luck with the planning, there are lots of people who have walked from Lisbon if that's what you're planning. A couple of forum members are walking the Interior from Viseu this fall and will have some info to share before you leave, but won't of course know anything about weather in winter. Bom caminho, Laurie