January 7 & 8 is the marathon weekend. Figure even if some folks stretch it out to a long weekend, the busiest time will be January 5-10. And it's no where near as busy during the marathon weekend as it is during school breaks or the summer. Instead of having a 0-5 minute wait for most attractions, during mareathon weeknd you may have a maximum 15-20 miute wait at some of the bigger attractions.
We were actually there during MLK weekend one year, and the MK was so crowded after the fireworks/SpectroMagic parade that they opened some side gate to let folks out. One CM said that that particular day had the record for attendance in the MK. He said there were more people there than had come in for New Years just the week before.