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, these were due on July 15 and Sept. 15. There normally would have been two additional payments for the first and second quarters on April 15 and June 15, but the IRS pushed them back due to the coronavirus pandemic. The fourth and final estimated payment for the 2020 tax year is 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, for corporations operating on a calendar year. 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 fiscal year, if they're on a fiscal year rather than a calendar year.
The IRS had not yet announced 2021 filing dates for estates, trusts, gift tax returns, exempt organizations, foreign partnerships, and nonresident aliens as of October 2020