One of the biggest turning points in Western, if not world, history occurred on October 31, 1517. Today, many churches from a reformed or semi-reformed tradition celebrated Reformation Sunday to commemorate Martin Luther's dispute with the Pope Leo X. While there is dispute over whether Luther actually nailed them to the door of the Castle Church in Wittenburg, there is little doubt that Luther had Ninety-five Theses or disputations against Leo.
Some may wonder what this has to do with American history. Just a quarter-century prior to Luther, a Genoese sailor by the name of Christopher Columbus claimed the New World for Spain (except for Brazil, which went to Portugal according to the Treaty of Tordesillas). While Columbus did not exactly discover America, since his voyages began continuous interaction and the famed "Columbian Exchange" his trip is worth remembering. Spain and Portugal were both intensely Catholic nations, and after the breakup of Catholic hegemony in Europe, controversy was bound to erupt as other nations attempted to rival these early explorers.
While England was still within the fold of Rome when Giovanni Caboto claimed parts of North America in 1497, it was after Henry VIII's break with Catholicism that the Reformation became important for current Americans. Although Henry just wanted Catholicism without the pope, a more Protestant England emerged permanently under his daughter Elizabeth I. It was under Elizabeth and her successor James I that England began attempting permanent settlements in the New World, partially as a part of this rivalry with Spain and other Catholic powers. The more radical Protestants in England, the Puritans established Massachusetts, and the traditional Puritan lifestyle of early New England began around 1630 (partially due to persecution from James' son Charles I and his archbishop William Laud).
Had Luther not complained about the selling of indulgences in Wittenburg back in 1517, the American religious landscape would be radically different than it is today. Therefore, whatever one's religious belief, it cannot be argued that October 31 has little significance for the history of Christianity.