I think it's more when the Japanese have a 3 day holiday in January. Usually it's the same weekend, but can change. Apparently mjm's boss didn't like the idea of 2 - 3 day weekends in advance as was the case in 2005
I just did some googling on Japan holidays and apparently the second Monday of January is a holiday for the "Coming of Age." So looking at a 2006 calendar it looks like January 6-8, 2006!
http://www.japan-guide.com/e/e2280.html