Do you mean the one in the US or in other countries, but they are all busy during festivals.

To the last answer, I would add that the 2-3 weeks following the Thanksgiving week will give you a chance to see the Christmas decorations without all the crowds. I find that those are some of the best weeks of the year to see the parks - with the caveat that the temps can vary pretty dramatically from the 40's to the 80's.

Thanksgiving is a wonderful time at Disney World; crowds are smaller than Christmas or Fourth of July, weather is generally beautiful (lower humidity and temps), but the optimal time for a visit is early October when school is back in session and no official vacation time.