December 10
Individuals: Reporting November tip income, $20 or more, to employers (Form 4070).
December 15
Corporations: Paying the fourth installment of 2025 estimated income taxes.
Partnerships: Paying the fourth installment of 2025 estimated income taxes.
Tax Exempt Orgs: Deposit Estimated Tax for 4th quarter due on Unrelated Business Taxable Income for Tax-Exempt Organizations. Use Form 990-W to determine the amount of estimated tax payments required.
December 31
Employers: Establishing a retirement plan for 2025 (generally other than a SIMPLE, a Safe-Harbor 401(k) or a SEP)