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First and foremost - you should have tire snow chains for your car, even if your car generally does no need them. Weather can turn very quickly in Yosemite - quickly enough that 4-wheel drive with snow tires will need chains. Check the weather forecast just before you come, and call 209-372-0200 (NPS phone number) for park road information. Note that rental car companies will not give you chains, however you can rent chains at various towns just outside the park.
Since you are staying at Curry Village, be prepared for very chilly weather. Its location makes it the coldest lodging spot in Yosemite Valley. Also, although bears are less active in the winter, take steps to ensure that your scented items (food, trash, toiletries) are locked up in bear-resistant areas - there is the occasional black bear that likes to stroll through in the winter.
Once you are here - shuttle busses are the best way to get around in the valley. They visit most spots on the east end (near Yosemite Falls and Half Dome). They do not go out to the west end (Bridalveil and El Capitan) in the winter. They come every 20-30 min.
Download a park newspaper from the NPS Yosemite website (
http://www.nps.gov/yose)
just before you come to help with more planning - it will be released around Dec 14th for the time of your stay.
As your stay is between Christmas and New Years, be prepared for a number of people in the Valley. While it is not nearly as busy as summer, that time is probably the busiest of the winter.
As for things to do - plan for winter activities. Most trails will have snow and ice, so best to plan around that. Tioga Pass and the Glacier Point area have heavy snow and are not auto accessible. The Mariposa Grove road is also closed, but hikeable (sometimes snowshoes are needed) - definitely worth the 2 mile hike!
For Winter Sports - Head to the Curry Ice Rink. They have several sessions per day, generally after 12. You can rate both figure and hockey skates while you are there. Badger Pass Ski area should be open - head to the DNC Parks and Resorts @ Yosemite website (
http://www.yosemitepark.com)
to learn more about rates and information. There are free shuttles which head up to Badger about 2-3 times a day (and will take you back as well). While it limits how you travel, it does save the hassle of driving - particularly if the weather is bad. Note that BOTH the Ice Rink and Badger Pass can close if the weather is bad. You can talk to the Valley tour desks once you are here for the lastest information.
Above all - bring a camera. Some of the most spectacular photographs you will ever take can be found in the winter - anything ranging from the light of sunset, to the ice of the Yosemite Falls cliffs, to the unexpected wildlife that crosses your path.
Enjoy your visit and remember to keep up to date on weather conditions!