When you say, “This Day,” what does that mean?
Such uncertainty’s nearly obscene
How did they name the date?
How can we keep them straight?
Seems we will e’er be caught in between

Event: Pope Gregory XIII implements the Gregorian Calendar. Thursday, October 4 was the last day under the Julian Calendar (the next day was Friday, October 15).
Year: 1582
Learn more: http://en.wikipedia.org/wiki/Gregorian_Calendar