Things to consider:
Many moving Web sites suggest moving in spring or late fall, with the rates being lower and moving companies not being as busy. Weekend rates also tend to be higher, rather than during the week days.
Working around the school calendar is also a good idea. If you are moving to a new area that involves children changing schools, you might reconsider moving directly following the end of school. This does not give your child time to adapt to the idea of moving and getting to say goodbye to their school, friends and neighborhood.
If moving in the summer, allow a few weeks in the new home to get accustomed to the new environment before the kids start back at school.
Another thing to consider, is to avoid moving before major holidays, since part of many holidays is being with family and friends. Give your family enough time in the new place before the holiday season begins to allow for meeting new people.