The Google Update, known as a massive algorithm (algo) shift is a myth. These dates are times that Google did a deep crawl of the web, and applied the core algo, It is done more or less four times a year. Because the deep crawl keeps in consideration things that do change, such as time on the web, and traffic levels, a site that had been high in the rankings, can plummet, as they loose points for traffic level decline, or when sites with more longevity points, have a huge increase in traffic. It can also change ranking because Google found less pages on one site, and more on others, thereby losing points in the points size element.
