The line between pre-Columbian and post-Columbian history in the New World is October 12, 1492 CE (using the calendar conventions of the time that have shifted a bit since then), the day the Christopher Columbus and his small fleet arrived in the Americas after the maritime trip from Spain, financed by Queen Isabelle, across the Atlantic Ocean. This set off a cascade of dramatic events in the Americas that have forever changed these two continents.
The year is also notable because in this year, the Reconquista was completed and the Muslim Moors were expelled from their last redoubt in Southern Iberia.
The Renaissance was in full swing at this point. The Protestant Reformation's beginning is conventionally dated to 1517 CE. In England, Queen Elizabeth I would go on to take the throne on November 17, 1558 and her fleet's defeat of the Spanish Armada, thirty years later, in 1588, would stun Europe.
This is recognized as "Columbus Day" by the federal government in the United States, and many other governmental entities in the U.S. and elsewhere in the Americas (observed this year on Monday, October 13, 2014 by the federal government since October 12, 2014 is a Sunday).
The modern significance and politics of this holiday in the United States is beyond the scope of this blog. But, this event itself continues to be arguably the most significant one in the history of the Americas.