I've been to Tenerife for the last 3 Xmas' (Playa de Las Americas) and have found it to be pleasantly warm during the day (low to mid 20's).
During the night I was quite comfortable walking around in jeans and t-shirt.
December is the wettest month though, followed by November and January. If you're going for a week, you might get lucky and it might not rain a single day, but from past experience, expect a bit of rain for a day or so.
http://www.holiday-we...om/tenerife/averages/