Planting date is an important consideration for soybean producers, and though recent studies have shown that planting in early May or late April planting can help maximize yield, responses to planting date are not consistent.  We investigated consistency of planting date response over 18 Illinois site-years, with planting dates from mid-April through early June.  Across nine sites with average yields greater than 3,800 kg ha^-1, the response of yield to planting date was curvilinear, described by: yield (kg ha^-1) = 5,052 - 0.266x², where x is days after April 1^st.  Across the nine lower-yielding sites, yield did not respond to planting date, and averaged only 2,520 kg ha^-1.  These results show that late planting can be a major yield-limiting factor under very favorable growing conditions, but tends to have little effect on yield under less-favorable conditions. With seasonal growing conditions typically unknown before the season, sound management should include early planting when possible as a way to maximize yield potential.