January 1 will probably be packed. After that you can expect the crowds to decrease some each day. "small world holiday" *should* be up at least through the first weekend of January, and, based on past history I'd expect it to be available through the second weekend, also.
we have been 3 times in that time frame, and IASW always has big crowds......but the holiday overlay is worth the wait! We usually ride it as one of the last things each night. Do it a few times early into your trip to hedge your bets. Also prepare to take lots of pictures in the plaza, just breathtaking especially at night.
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