Deadlines for annual returns are almost invariably in the spring, with extensions available until the fall:

• Individual income tax returns: April 15 falls on a weekday in 2021, and it's not a holiday this year, so the filing deadline for your 2020 personal tax return is indeed April 15, 2021.
• Estimated tax payments for the 2020 tax year (IRS Form 1040): For 2020, the first and second payments were both due on July 15. Third quarter was due October 15th. The fourth and final estimated payment for the 2020 tax year was due on Jan. 15, 2021.
• Partnership returns (IRS Form 1065): These are due March 15, 2021. The extended deadline is Sept. 15, 2021.
• C-corporation income tax returns (IRS Form 1120): These are due April 15, 2021, for C-corporations that operate on a calendar year. The extended deadline is Oct. 15, 2021. The deadline for C corp returns is the 15th day of the fourth month following the end of the corporation's fiscal year if the corporation operates on a fiscal, rather than a calendar, year (Ref 2).
• S-corporation returns (IRS Form 1120-S): These returns are due March 15, 2021. The extended deadline is Sept. 15, 2021. The deadline for S-corp and partnership returns is the 15th day of the fourth month following the end of the year.
• Foreign partnership returns are due on the same date as other partnership returns March 15, 2021.
• Trust, estate and gift tax returns are due April 15, 2021.
• Nonresident alien returns are due April 15
• Exempt organization returns are due May 15, 2021

Deadlines Organized by Date

Here are important dates in chronological order, if you prefer to just flip through and mark them on your calendar. They include some additional to-do-by dates that might apply to you.

• March 1, 2021: Deadline for businesses to mail Forms 1099 and 1096 to the IRS
• March 2, 2021: Deadline for farmers and fishermen to file individual income tax returns unless they paid 2020 estimated tax by January 15, 2021, in which case the filing deadline is April 15, 2021.
• March 10, 2021: Deadline for employees who earned more than $20 in tip income in February 2021 to report this income to their employers
• March 15, 2021: Deadline for corporate tax returns (Forms 1120, 1120-A, and 1120-S) for tax year 2020, or to request an automatic six-month extension of time to file (Form 7004) for corporations that use the calendar year as their tax year, and for filing partnership tax returns (Form 1065) or to request an automatic six-month extension of time to file (Form 7004)
• March 31, 2021: Deadline for businesses to e-file Forms 1099 and 1096 to the IRS, except Form 1099-NEC
• April 12, 2021: Deadline for employees who earned more than $20 in tip income in March 2021 to report this income to their employers
• April 15, 2021: Deadline to file individual tax returns (Form 1040) for the tax year 2020 or to request an automatic extension (Form 4868) for an extra six months to file your return, and for payment of any tax due
• April 15, 2021: Deadline for household employers who paid $2,200 or more in wages in 2020 to file Schedule H for Form 1040
• April 15, 2021: Deadline for first-quarter estimated tax payments for the 2021 tax year
• May 10, 2021: Deadline for employees who earned more than $20 in tip income in April 2021 to report this income to their employers
• June 10, 2021: Deadline for employees who earned more than $20 in tip income in May 2021 to report this income to their employers
• June 15, 2021: Deadline for second-quarter estimated tax payments for the 2021 tax year
• June 15, 2021: Deadline for U.S. citizens living abroad to file individual tax returns or file Form 4868 for an automatic four-month extension
• July 12, 2021: Deadline for employees who earned more than $20 in tip income in June 2021 to report this income to their employers
• Aug. 10, 2021: Deadline for employees who earned more than $20 in tip income in July 2021 to report this income to their employers
• Sept. 10, 2021: Deadline for employees who earned more than $20 in tip income in August 2021 to report this income to their employers
• Sept. 15, 2021: Deadline for third-quarter estimated tax payments for the 2021 tax year
• Sept. 15, 2021: Final deadline to file partnership and S corporation tax returns for tax year 2020, if an extension was requested (Forms 1065 and 1120-S).
• Oct. 12, 2021: Deadline for employees who earned more than $20 in tip income in September 2021 to report this income to their employers
• Oct. 15, 2021: Final extended deadline to file individual and corporate tax returns for the year 2020 (Form 1040 and Form 1120).
• Oct. 15, 2021: Deadline for taxpayers who earned $69,000 or less in adjusted gross income (AGI) for tax year 2020 to use Free File to prepare and file their returns
• Nov. 10, 2021: Deadline for employees who earned more than $20 in tip income in October 2021 to report this income to their employers
• Dec. 10, 2021: Deadline for employees who earned more than $20 in tip income in November 2021 to report this income to their employers

Sourced from TheBalance.com