The first known income tax that Americans were legally required to pay was enacted during the 1860s, and the Presidency of Abraham Lincoln. The Civil War was proving very costly to fund, and the President and Congress created the Commissioner of Revenue and enacted a law requiring citizens to pay income tax.
Originally, the deadline for completing and filing your individual income tax was not April 15th. In the beginning, it was first set for March 1st. Then, during 1918, Congress pushed the date out to March 15th. Then, in the great overhaul of 1954, the date was once again moved forward to April 15th, and this is where it remains today. But, it has only been set this way for a little over 50 years. Thats not very long, in historical terms, and it could possibly be changed again.
If you are an individual tax payer, you are required to file either a return or an extension of time to file (Form 4868) by April 15th. Corporate and other legal entities are required to file their tax return by March 15th, and if not, they also must file an extension of time to file. What this extension does not do, is to extend the amount of time you have to pay any taxes due...