Google celebrates its birthday every year on September 17th. Why this day? Because they found this to be a good day, and since they had to choose a day for their birthday, they chose it. :) By the way, Google was registered as a company on September 4, 1998, but the Google.com domain was already occupied on September 15, 1997 by the two founders Sergey Brin and Larry Page. It also shows that the day of their birthday was chosen in a striking way, as it is not correct either with the date of the domain reservation or the date of the establishment of the company. In any case, if we take the year the company was founded and September 17, as defined by them, Google will celebrate its 19th birthday on September 17, 2017. :)