The deadline to file Form 1120S for the tax year 2023 is March 15th, 2024. However, if you need an extension, you can file Form 7004 to get an additional 6 months, moving the deadline to September 15th, 2024. It's important to note that these deadlines can vary if they fall on a weekend or a holiday, so it's always recommended to double-check with the IRS or a tax professional for the most accurate information.