Budding is most commonly done when there is

Budding is most commonly done when there is good sap flow in the plants.
Budding is inserting a single bud from a desirable plant into an opening in the bark of a compatible rootstock to create an advantageous variety (cultivar) and rootstock combination.
Vegetative buds are used as a scion. The time of budding depends upon the availability of buds and cell sap flow in the rootstock seedlings so that bark separates easily to take scion bud.