I am not sure when the tree sets its buds, but you would trim the tree after the tree has bloomed in the spring to avoid cutting off the buds for the flowers.
When do you prune it?
Do'nt prune it in the winter. Cut it back in the Spring just as growth starts as hard as you like.
winter
During the Winter.
If you prune a Forsythia in Winter you will cut off the following Springs flowers. Prune a Forsythia after it ha s flowered.
Leave the dead flower heads on all winter, prune after the risk of frost has passed in the spring.
Dogwood Winter
It is not recommended to prune chrysanthemums in early winter. It is best to prune them in spring and remove all the dead parts. However, you should water them if the weather is above freezing.
Many believed that the best time to prune Raspberry bushes are in the late winter or early spring. The plant should be over a foot before trying to prune the bush.
Best to prune outdoor juniper bushes in early spring(March or April in the northeast US) to allow ample time for regrowth before winter sets in. If you prune too close to winter, there's not enough time for the regrowth to occur and the bush could be damaged.
in the carppy day on the sun set of winter
Yes. Cornus are deciduous so they drop their leaves in the Autumn.