If you're in the position to pick and choose when you move house, you're in a very lucky minority! Most moves are dictated by the time it takes to sell your property, to find, agree a price on and exchange contracts on your new home, and then your chain remaining in place to allow you to complete.
If you're in control of your move date, the best times are either late spring or just after the summer holidays, when most moving companies are less busy and consequently their rates will likely be lower as well. Weekend rates tend to be higher also, as are the associated costs for renting crates, storage space and truck hire.
However, most of us have other factors to consider such as kids, wives/husbands, jobs, schools and a whole host of other reasons why our move date is often dictated to us, rather than decided by us.