Hi Sandra, Sorry for the delay in responding. The survey can be performed at any time but some trees will be easier to identify than others at different times of the year (e.g. conifers will be easier to identify during the winter as they will still be in leaf whereas deciduous trees will have shed theirs).
The answer, therefore, is the best time is whenever you are able to do it. However, from a personal perspective, the greatest pleasure would probably occur during the blossoms of spring or the vibrant colours of autumn.