Whenever your weather gets cool and damp (comparatively anyway) - presumably you do have spells of cooler weather over autumn/winter? In UK the answer would be in winter, while the tree is dormant but when the ground isn't frozen.

thankyou,Bamboo. Thats wot i thought. were still getting temps of up to 29 deg,so i shall wait untill the cooler weather arrives. the leaves havnt yet changed colour,so presumably after they fall? we havnt had rain since late may!! ground is similar to concrete!

You need to pick the coolest period you get, when hopefully the soil will be nice and damp.

Hi bamboo,thankyou for your helpful advice.Your comment on the ground being 'nice and damp'almost never happens! yes, we do get rain,but not often,it was may when we had the rain last.However, in Jan we get frosts [sometimes down to -6 ] and the ground is still hard and dry.Do you advise giving the area a real long soak, before planting? even in winter? our main climatic problems are winds [ sometimes up to tornado levels]& ground frosts in winter. We have no natural wind-breaks.

Yes, I do advise giving the ground a good soaking before you move the tree - soak the area the tree is currently inhabiting, and the area in which you intend to plant it. And you will need to keep the tree well watered for a year or 2 after moving it, a good couple of gallons a week.