Our US Federal income tax system works on a pay-as-you-go basis. As you earn income, you must pay taxes (sorry). Therefore, April 15 is not the “due date” for your tax payments. Rather, April 15 is the date by which you must “square out” with the feds. In your tax return, due April 15, you figure out the real taxes due. If you already paid more than that throughout the year, you will get a refund. If you paid less than that throughout the year, then you need to pay up the difference. State income taxes work the same way.
Benjamin Franklin said, “In this world nothing can be said to be certain, except death and taxes.”
If you are required to pay estimated taxes, plan carefully for them.