Fall tilling in the garden and annual flowerbed is the best time. When you are ready to plant in April with the early spring crops the soil is ready with reduced weed interference.
In the fall the soil is dryer and will compact less. Add organic matter to be dug down like peat moss, compost, or rotting straw, hay, alfalfa matter. These materials will decompose over the winter helping build stronger plant roots in the spring.
Tilled soil also helps control disease and weeds. Also, the garden will catch snow improving soil moisture for the spring.
You should not till more than once a year because it breaks down the soil particles too much especially for clay soils. Many gardeners now plant using the built-up method (no-till) walking only on walkways. Just make sure you remove any disease plants or weeds from the garden.