Usually its a Friday, Saturday, and Sunday event during the first two weeks of December. The last two weeks of December through the first week of January its every night.
To give you an idea of what to expect, here are the 2014 dates:
Our Dreams Unlimited Travel Agents will assist you in booking the perfect Disney getaway, all at no extra cost to you. Get the most out of your vacation by letting us assist you with dining and park reservations, provide expert advice, answer any questions, and continuously search for discounts to ensure you get the best deal possible.