I filed my S Corp taxes on 04/11 but received a $245 fine and letter from the IRS saying my taxes were filed late. Is this a mistake or am I missing somthing?

Is this a mistake or am I missing something?

You missed something. 2024 calendar year S corporation returns were due March 17th this year.

See https://www.irs.gov/instructions/i1120s#en_US_2024_publink11515kd0e416

S-corp and Partnership returns are always due on March 15 instead of April 15, unless you get an extension. I believe the extension is requested on form 7004.

https://www.irs.gov/forms-pubs/about-form-7004

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@Opus 17 wrote:

S-corp and Partnership returns are always due on March 15 instead of April 15.....

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For calendar year entities, right. Otherwise, by the 15th day of the 3rd month following the end of the entity's tax year (so not always due on March 15th).