The Christmas Pack added Christmas themed decorations to all players' cities for 1 month. All City Halls received massive Christmas trees and all streets had (unfortunately 8 sided) snowflake decorations. Night-lighting from cars and streetlamps was added also. The Christmas Market and the Christmas Park were two special buildings that came with the pack.
An update in late January 2010 turned off all decorations but kept the night lighting. The exclusive Christmas buildings were converted into brand new, generic buildings but retained their decorations inside the buildings (viewable if camera is inside building).
There was an associated contest here: http://www2.citiesxl.com/index.php?option=com_content&view=article&id=167%3Agreat-holiday-contest&catid=3%3Anewsflash&lang=en