If you have a well-stored scions, it can be done basically the whole year. In summer it's best to use a type of bud grafting (chip budding also called).

You can do a late grafting with last years scion and the bud will develop in that same season. You can also wait for August and collect a newly formed this year sleeping bud and graft that and it will develop next spring.

Also walnuts and other nut trees are usually grafted in warm times, window grafting usually (not sure how it's called in English, it's where you cut a rectangular piece of the bark containing a bud).