Doing a day trip anyplace on December 25 isn’t going to work as most everything is closed Christmas Day. And a lot of places close early on the 24th, as this is the day most Portuguese really celebrate the holiday.
I would suggest you visit Evora as early in your stay as possible and consider dropping Obidos. Spending both the 24th and 25th in Lisbon will be best, as you can walk around and enjoy the city even if a lot of things are closed. The public transport will still be running to help you get around.
Also on the 25th most restaurants are closed. You will find the Indian restaurants open and hotel restaurants, but you might want to do your research as to where you will eat on Christmas Day.
And, the biggest issue with the weather that time of year is more chance of rain. Depending on where you are from, the temperatures here will possibly be an improvement. And, there are a lot of very sunny, fairly warm days that are glorious in the winter.