This is the 100th episode.
It was confirmed by Rebekah that it has been almost three months since I Know What You Did Last Summer, placing the events in this episode in the late November 2011 or early December. This means that the The Originals is on the same timeline as well.
"500 Years of Solitude" is a reference to the book One Hundred Years of Solitude by Gabriel Garcia Marquez. In this case, it refers to Katherine's time on the run from Klaus.
The script was finished on October 17, 2013 while the filming started on October 28.
Nina Dobrev, Paul Wesley, and Ian Somerhalder are the only actors to appear in all of the first 100 episodes.