Taxpayers who do not have sufficient tax withheld by payers, (e.g., employers or pension plans, etc.), may be required to pay an estimate of their current year tax liability, in installments according to the following “quarterly” schedule. As you can see, technically, the schedule is not strictly quarterly.
Income Period Payment Due Date
Jan – Mar (3 months) April 15
Apr – May (2 months) June 15
Jun – Aug (3 months) September 15
Sep – Dec (4 months) January 15 of the following year.
If the due date for making an estimated tax payment falls on a Saturday, Sunday, or legal holiday, the payment will be on time if you make it on the next day that’s not a Saturday, Sunday, or legal holiday.
Kenneth Athon Tax Strategies 