According to the most popular version of the story, Pipp showed up at Yankee Stadium that day with a severe headache, and asked the team's trainer for two aspirin. Miller Huggins, the Yankees' manager, noticed this, and said "Wally, take the day off. We'll try that kid Gehrig at first today and get you back in there tomorrow." Gehrig played well and became the Yankees' new starting first baseman. This story first appeared in a 1939 New York World-Telegram on Gehrig's career, in which Pipp was interviewed. Pipp was later quoted to have said, "I took the two most expensive aspirin in history."
But I know there are other stories out there, the bottom line is he took the day off for whatever reason, and the kid got his job.