It depends on what is growing there now. If it was me I would wait until July, but if the plot has some grass such as fescue, you might want to go a little sooner. If this summer you get a good rain, followed by a hot dry spell, I would disk as soon as the ground was dry enough. One other option especially if it has got grass or other deep rooted weeds, is to spray it with Glyphosphate, and to be honest that might be your best bet. If done that way you could wait a lot later to disk, possibly 2 weeks before planting and end up with a much better plot than disking alone. I would spray 4 to 5 weeks before planting. Just my opinion.