Tax day is normally April 15th each year, but the date can actually change by a few days so it is important to keep up to date so as not to miss the deadline for filing; otherwise, an extension will need to be filed.
Some interesting background information is that 3 percent was taxed on income of $800 per year back in 1861. The monies gained was to fund the American Civil War effort.
Income tax was considered unconstitutional and successfully repealed in 1872.
In 1874, a 2 percent personal income tax was introduced under the Revenue Act of 1894, but the legalities were still unclear.
"In 1913, the "Sixteenth Amendment" to the Constitution of the United States was ratified. This cleared the way for the modern income tax system in the USA. The details of the income tax system have changed greatly since 1913. The top rates of tax have varied enormously and were particularly high during the First and Second World Wars and the Great Depression. Individuals and families with very low levels of income do not have to pay income tax and may receive some subsidy via the tax system."
Source: time and date