There are some great online design resources for calendars, including ones where you can print your own calendars, planners and greeting cards. This Advent Calendar by mibostudio is only one of a few free items they have available for download.
Why is it green? Print it at home on recycled paper and save on shipping and transportation costs.
mibostudio offers a free 2010 planner and free Advent Calendar.
Other printable goodies are available at Gleaux including a full printable 2010 calendar (PDF link).