In 1904, the Boston Herald refers to NY baseball club, called the Highlanders as "Yankees", when it reports "Yankees take 2," The Yankees did not become the team's official name until 1913.
So let's go back to the beginning...
In 1900, Ban Johnson, the president of the Western League, a minor league changed the name to the American League and asked the National League to classify it as a major league. Johnson offered that the new AL would operate on a friendly basis but the idea was rejected by the NL. In 1901, Johnson went ahead with his plans and the American League was born.
The new AL wanted to add a team in New York City, but plans were blocked by the NL's New York Giants. So, a team was instead placed in Baltimore, Maryland, named the Orioles. The team was managed by John McGraw, but he soon had a falling out with Johnson so he quit the Orioles and went to manage the NY Giants, taking many of his best players with him. The Baltimore Orioles barely made it through the season and afterwards were disbanded.
By 1903, the AL and NL had worked through their issues and Johnson requested that the AL be allowed to have a team in NYC. The NL owners agreed and the new franchise was awarded to Frank J. Farrell and William S. Devery who relocated the defunct Baltimore Orioles to NYC. The new "Hilltop Park" was built in an elevated location in upper Manhattan, which earned the new team the nickname, the Highlanders. The team was also referred to as the New York Americans (there was a Boston Americans team) or the Invaders. However in 1904, a sports reporter in New York referred to the team as the "Yankees" or "Yanks" for short, and that was followed up by the similar reference by the Boston Herald.
In 1913, the NY Highlanders lease with Hilltop Park expired and rather than renew, they opted to share the Polo Grounds with their cross town rivals, the NY Giants. The name "Highlanders" fell into disuse among the press so in 1913, the team officially became known as the New York Yankees.
Now, I could talk about my love for my adopted team, the NY Yankees, and my hatred for my once childhood team, the Boston Red Sox (born in 1986 in a dorm room in New Hampshire after a fateful loss to the Mets). Or the time I had the opportunity to have a couple of beers with the 1961 Yankees (sans Maris and Mantle of course) at the Hilton in Teaneck New Jersey. How I look for Mariano whenever Sandman comes on, and "start spreading the news" after every win. Or the many games I've been lucky enough to attend...but how I gave away tickets to the now infamous "Aaron F'ing Boone" game.
Instead, I will talk about the famous pinstripes.
The NY Yankees (then the Highlanders) wore pinstripes in 1912, but only for one season. They re-added them on a permanent basis soon after Jacob Ruppert bought the team in 1915. Though I do like the rumor that the team added pinstripes to their uniforms in order to make Babe Ruth look thinner.