When a major event occurs, or you make a big change to your site or your marketing, it’s always a good idea to make an annotation in Google Analytics. That way, in the future, whoever is looking at the stats may be able to see a correlation between what took place, and how it affected traffic.
Did you have the site redesigned? Maybe you ran a TV or radio ad campaign? Was there a major Google algorithm update? Did a huge storm batter your area and knock out the power? Whatever event might have taken place, it only takes a minute to add an annotation in Google Analytics, and this 2 minute video will show you how.