Is it better to dig soils in the spring or the autumn?
It is better generally to do tasks like digging, when you feel like doing it. If you don't feel like doing it, then you can practice the 'no-digging' method of crop production. It works!
If your vegetable plot soil is heavy - or clay - then if possible, it can be rough dug in the autumn to allow the winter weather conditions to break the soil down. Frosts really help with this. The following Spring, you should have a soil that is easier to work than if it had not been Autumn dug.
If you have a light to sandy soil, then simply spread some manure over your soil in the autumn and dig what is left into the ground in the Spring. The covering of manure will help the lighter soils being battered by winter weather.
Some writers, will suggest that the soil should be double dug (hard work) in the winter. It doesn't happen in nature, and I have never experienced any negative results by not doing it. If you want to get fit, then do it by all means. For us, it is not a necessary thing to do for good fertile soil. far better to cover with a layer of compost and let the worms do the job!