Malta has a typically Mediterranean climate, with hot, dry summers and cool, wetter winters. In comparison to northern Europe, the climate is much milder in Winter, and December in Malta can be quite pleasant averaging 5 hours of sunshine per day.
Typical daytime temperatures in December are from 15-20C, so many people are still in t-shirts on sunny days. There should be some lengthy sunny spells, however it can rain so pack an umbrella. After sundown, which is at around 5pm in winter, temperatures can drop to around 10-12C, so pack trousers, socks and knitwear for the evenings.