Hi Pixie,
The official tourism season is from Easter to mid-September however Portugal is regarded as an all-year destination being one of the warmest European countries. Even in December you will find that temperatures are mild rather than cold but it is likely that you will experience some showers. Temperatures are likely to be around 15°C.
I would take comfortable clothing that is smart/casual. I would perhaps take something more formal for the Portugese style Christmas Eve dinner and the festive Christmas dinner. People always seem to like getting dressed up at Christmas! I know I do. As temperatures won't be terribly cold don't pack too many woolies but I would take a few cardigans or jumpers for cool evenings and a lightweight jacket.
The currency is Euros and as Portugal is part of the European Union no visa is required. The official language is Portugese but as Portugal's economy is very much geared towards tourism you will find the vast majority of people also speak English. The length of the flight is approximately 2 hours 30 minutes.
Hope this information helps. My biggest piece of advice would be to just relax and enjoy your holiday. Make the effort to get to know your group and you'll have a fabulous time and hopefully make some new friends. Let me know if you decide on this holiday.
Warm wishes
Cheryl, Your Editor