During Christmas holidays people tend to become a lot more careless about the environment and the way they use the products. A lot of electricity is being wasted, much more food is being cooked and eventually thrown away and a lot of gifts are exchanged without paying any attention on their usefulness or whether they satisfy any sustainability parameter (materials, batteries, end-of-life, etc.).
To begin with, one of the most important sources of waste of energy is the Christmas lights. In the past, the ‘best’ house was the one with the most decorative lights. Nowadays, the electricity bill has extremely risen and electricity itself is the largest factor of spending the natural resources. Few simple things that can be done to celebrate Christmas and feel happy, but at the same time be careful with the waste of energy, are the reduction of outdoor lighting and the use of LED lights for the decorations. LED lights use up to 95% less energy than the traditional ones. Furthermore, all lights should be turned off when it is bedtime, as nobody is up any more. A timer can be used for that reason in order to avoid forgetting them on. , 
In addition to this, we can also buy a live Christmas tree in order to avoid using plastic trees made out of petroleum products (PVC). Even though some may believe that by using plastic trees we save the life of a tree and we can use it for many years, that is not completely true. A plastic tree contains a lot of embodied energy to be manufactured and shipped and at the end of its life it will end up to landfills, where its plastic content will last forever. On the other hand, the live tree grows on tree farms and is replanted regularly. What is more, one can replant the tree to larger pots or eventually to the garden when it gets too big. Thus, it can be used for years. If there is not much space, then it can be used for the fireplaces and the mulch for gardening. Having a live Christmas tree in the house gives the smell of Christmas and is the most sustainable way of celebrating. 
Some more alternatives for a sustainable Christmas tree are shown in the pictures:
1. How to have a ‘green’ Christmas: http://eartheasy.com/give_sustainchristmas.htm
2. 10 Tips for a Sustainable Christmas: http://denmark.dk/en/meet-the-danes/traditions/the-christmas-month/10-tips-for-a-sustainable-christmas/