The IRS will begin accepting individual tax returns for the 2018 tax year on January 28th, 2019.
If you have a business return (1065, 1120, 1120S) to file, the IRS has already began accepting those returns.
Partnership (1065) and S Corporation (1120S) returns are due March 15th,2019. You may file an automatic 6-month extension which will allow you up until September 15th, 2019 to file these returns.
Individual (1040), C Corporation (1120) and Trust (1041) returns are due April 15th, 2018. (Note residents of Maine and Massachusetts have until April 17th ,2019 due to Patriot's Day celebrated in these two states on April 15th and the federal holiday of Emancipation Day on April 16th.) You may file an automatic 6-month extension for these returns as well, which will allow you up until October 15th, 2019 to file your return. IMPORTANT NOTE: an extension to file does not extend your time to pay any tax due. Penalties and interest will accrue on taxes not paid by the April 15th deadline.