Individuals: If you are not paying your 2018 income tax through withholding (or will not pay in enough tax during the year that way), pay the third installment of your 2018 estimated tax; use IRS Form 1040-ES.
Employers: For Social Security, Medicare, withheld income tax, and non-payroll withholding, deposit the tax for payments in August if the monthly rule applies.
Corporations: Deposit the third installment of estimated income tax for 2018; use the worksheet IRS Form 1120-W to help estimate tax for the year.
Partnerships: File a 2017 calendar-year return (IRS Form 1065); this due date applies only if you timely requested an automatic six-month extension, and provide each partner with a copy of their final or amended Schedule K-1 (IRS Form 1065) or substitute Schedule K-1.
S corporations: File a 2017 calendar-year income tax return (IRS Form 1120S) and pay any tax, interest, and penalties due; this due date applies only if you timely requested an automatic six-month extension, and provide each shareholder with a copy of Schedule K-1 (IRS Form 1120S) or a substitute Schedule K-1