Many Americans will celebrate Monday as Columbus Day, a federal holiday that marks the anniversary of Christopher Columbus’s Spanish-led expedition arriving in the Americas, or as Indigenous Peoples Day, to acknowledge those who were displaced by European settlement in North America.

Others, however, will wait for Tuesday to celebrate something else: Leif Erikson Day, a celebration of the Viking explorer credited with reaching the continent around the year 1000, nearly 500 years before Columbus did.

But, while it may sound only fair to share the credit for exploration, the movement to recognize Erikson also has a dark back story, as Leif Erikson Day’s history is connected to nativist backlash against immigration to the United States. At one point, for some people, the debate over who really “discovered” America came down to one question: who was whiter?