IT now seems like ancient history. Forty years ago this summer, Martin Luther King, Jr., stood at the foot of the Lincoln Memorial and delivered his “I Have a Dream” oration. Jim Crow still reigned supreme, the country was seething with racial tensions, and it was not yet clear whether the federal government would act. Into this morass stepped King, who pointed a way toward racial equality and national unity. Twenty years later, in 1983, President Ronald Reagan signed legislation naming a federal holiday for King. But despite the holiday and the passage of time, King’s significance remains a point of contention. Do we really know what we are celebrating on the third Monday of every January?