This is the result of a spell cast by Jadis, the White Witch, when she conquered Narnia and reigned as a tyrant. This was in the Narnian year 900. The Witch's spells were broken in the year 1000, under Aslan's influence, and the winter came to an end.
So ... rather than saying that it was (or is) always winter but never Christmas, in Narnia, it would be better to say that it was always winter, but never Christmas, for a hundred years.
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