Christmas marks the winter solstice among some Christian families. Those families which still keep the Gregorian calendar often observe it on December 25, while others track the length of day and night to determine when to celebrate the holiday.
With less seasonal difference, Christmas does not have a strong association with changing weather so much as a change in the length of day and night. Most people in the Fifth World stay up for a few hours past sunset, and rise shortly before dawn. This means that they sleep longer in the winter than in the summer.
Less seasonal change has diminished the value of seasonal holidays marking those transitions, and the failure of agriculture has made the calendar much less useful as well. This has led many families to abandon holidays altogether, including Christmas. Due to its popularity among their civilized ancestors, though, some families have kept Christmas traditions alive nonetheless.