Soil temperature doesn't care what month it is. There is a reason most farmers till their soil in the fall. That dark soil absorbs all the suns energy in the spring drying and warming it up. If you are planning on using tillage and we get a early/warm spring you may be able to plant it much sooner than June.
We always need to ask ourselves what are our objectives. If that is to plant a brassica plot in early August 60 days on the buckwheat will be more than adequate. That puts you at June 1 planting date. If your worried about moisture plant it earlier. It just isn't going to put on much growth until the soil warms up. You do need some moisture for germination though so just use your best judgement.