Hope and Fall
Satan is the devil, he tries to make our lives painful and even worst when we die. If our world is only pain and evilness people will "adapt" to survive and support this and they won't suffer anymore (or at least they will suffer less) and Satan doesn't want that.
If he every year make gifts to all the people in the world he increases the hopes of people and also bring them happiness to survive in this world. I think this is "good" to him because he can make their lives even more painful, think about this: you are a poor person who live in an abandoned building, one day this building is destroyed and you lose your "house", this person won't suffer much because he is already suffering, but if this person was a rich and one day he lose everything he would suffer a lot, the "fall" would be bigger and Satan wants that. He brings hope and happinnes to us to suffer more and have a bigger "fall" when they die and come to the hell.
Also he can play with the "coal" gifts: you was waiting for all the year to your gift and in christmas you get a coal, this would be very disappointing.
Also you can use the christmas to make people sad indirectly, I am not sure but christmas is the day with more suicides because people suffer when they see families happy and they are alone.
Sins
Satan use christmas to increase the sins of people and corrupt their souls.
In christmas people are very:
- Greedy, they want the best gift.
- Envy, some people can't afford good gifts and other can, this persons has envy of the rich people.
- Gluttony, in a lot of countries there is a tradition of eat a lot of yummy meals and sweets in christmas, easter day and new year.
- Sloth, some people take the day free.
- Pride, people sometimes buy the most expensive gifts and decorations for christmas in order to show them to their friends.
- Lust, from Wikipedia:
[..] lust could also mean simply desire in general; thus, lust for money, power, and other things are sinful.
Peolpe want material things.
- Sadness, like I said in the Hope and Fall title some people are very sad in christmas for not have family or friends.
Reduce Charity
When Santa gives presents to poor persons other people (like richs) doesn't give presents because they have already one and charity is one of the seven virtues of Christianity, Satan is trying to eliminate one of the seven virtues of people.
Absolution
Satan is the evil, sorry but I don't read The Holy Bible so I don't know if this is true but God is all mighty, right? So... Why He doesn't destroy Satan? My idea is that Satan isn't so bad, every year Satan regrets his sins making gifts to all the people in the world and bringing them hope and happiness. By this way God can't "purify" Satan, because he regrets his sins every year.
Christ's attention
Christmas is the birthday of Christ but Satan create the "Santa" character to make Christmas about him and not about Christ. A lot of people doesn't know that Christmas is the birthday of Christ but they know about Santa.
Materialist people
Satan makes materials gifts to people instead of an "spiritual gift" like absolution, people try to be goods persons in order to get a material present in christmas and no the soul absolution. He make people more materialist.