http://www.irs.gov/publications/p505/ch02.html#d0e6371 When To Pay Estimated Tax For estimated tax purposes, the year is divided into four payment periods. Each period has a specific payment due date. If you do not pay enough tax by the due date of each of the payment periods, you may be charged a penalty even if you are due a refund when you file your income tax return. If a payment is mailed, the date of the U.S. postmark is considered the date of payment. The payment periods and due dates for estimated tax payments are shown next. For exceptions to the dates listed, see Saturday, Sunday, holiday rule below. For the period: Due date: Jan. 11 – March 31 April 15 April 1 – May 31 June 15 June 1 – August 31 September 15 Sept. 1 – Dec. 31 next year January 15