Leave it to TurboTax to put out a visual history lesson on the past 100 years of tax history -- particularly tracing the origin story of the April 15th tax day.
In their visualization, they note February 3rd, 1913 as the day Congress ratified the 16th Amendment giving them the authority to enact an income tax. Prior to 1913, only taxes were raised during the civil war and the first income tax day was March 1, 1914.
Four years later the the "Revenue Act" moved tax day forward two weeks, standing on the day we currently 'celebrate' today (April 15th). Here's a complete look at the history: