April 15th, otherwise known as Tax Day didn’t come into effect until 1955, where it has remains to this day.
With the Revenue Act of 1861 to help fund the civil war, the first income tax was introduced. It was subsequently repealed, re-adopted and ultimately declared unconstitutional.
In 1895, the Supreme Court heard Pollock v. Farmers’ Loan & Trust Co. who challenged the Wilson-Gorman Tariff Act which taxed incomes over $4,000 at the rate of 2%. The court ruled that the Act’s taxes were “direct” taxes which according to the Constitution, should be appropriated by the state.
In 1913, the 16th Amendment was added to the Constitution which gave the legal authority to Congress to tax all incomes. The filing deadline at that time was March 1st; changed to March 15th in 1918 and in 1955, changed to April 15th.