Christmas is celebrated in December because it's the anniversary of Jesus' birth. December 25th is the date that's been traditionally associated with Jesus' birth, although there's no historical evidence to support that date. The church chose December 25th because it's close to the winter solstice, a time when the days start to get longer again and the earth begins to warm up. It's also a time when people are more likely to be indoors, which makes it easier for them to gather together and celebrate.