Clay soils: are best dug in autumn, but avoid carrying out this task when the soil is wet and claggy. Autumn digging allows the frost to break up the soil over the winter, improving the structure.
Light, sandy soils: are best dug in spring. However, digging can be carried out from autumn to spring, as long as the soil is not waterlogged or frozen.
Because digging leads to moisture loss, complete it before the warm spring weather arrives.
Consider hiring a mechanical rotavator to do the cultivation for you if time runs short. Light soils can be handled by a two to five horse power model, but hard or heavy soil needs a larger model. Rotavating wet soil is extremely damaging; wait for drier conditions.