What is the best time to graft apples in zone 5 ?
When you see buds pushing it's the time of start, then you're good for about another month, using dormant material.
Bob, if you are new to grafting you should also try some summer grafting (budding). In Zone 5 you will do this in August, with buds collected immediately before you graft. I have had good success with this method. Of course, with apples, almost any method works.
Here, in SE NY, I tend to get the very best results from grafts I place on trees just before they start growth. I seems that the closer the growth of the graft and tree coincides the more growth one gets from the graft, all other factors being equal.
The relative vigor of the tree being grafted to is equally important. I no longer graft onto trees transplanted after previous growing season.
In zone 5 Kansas we make sure the scion wood is collected and kept dormant in the fridge until the sap starts to move and buds start swelling.