There is not a hard and fast rule to follow for this question. There are many factors that come into play including the type of fruit tree involved, your climate and weather conditions, and even the type of graft you elect to use.
The above noted, here are a couple general guidelines that may facilitate grafting success:
For deciduous fruit trees (those that lose their leaves annually), try to graft shortly before both the stock tree and the scion wood break dormancy.
For evergreen fruit trees (like citrus), try to graft when the buds of the stock tree begin to swell, but before they actually push out.
