Your first sowing can be after danger of frost has past. Most people till it in when it has been flowering, but before seed is produced. (You can actually produce your own buckwheat seed if you want, but I am not sure how to best harvest it.) It's not a problem if you till earlier, you just won't get the maximum benefit. It's not a problem if you till later, you just will have produced seed that will come up randomly in the garden where it falls. A very easy to pull "weed" and bees love the flowers!
