The first filing deadline is the end of the day on 15 Apr for most states. Taxpayers living in Maine or Massachusetts have until April 17, 2024, due to the Patriot's Day and Emancipation Day holidays. You can request an automatic 6 month extension of the filing deadline but must do so by the first filing deadline (15 Apr).
The extension is for for filing only not paying. Interest and penalties on amounts due will begin to accrue after the 15 Apr deadline.