Estimated tax payments (Form 1040-ES): Payments are due on the 15th day of the
4th, 6th, and 9th months of your tax year and on the 15th day of the 1st month after your tax year ends.
If the 15th happens to fall on a Saturday, Sunday, or a Government Holiday, then your payment is usually going to be due on the following Monday — but the actual due dates are also printed on the 1040-ES payment coupon that must accompany your estimated tax payment.
2012 IRS 1040-ES (Estimated Tax Payment) due dates are:
Payment #1 due on (or before) April 17, 2012
Payment #2 due on (or before) June 15, 2012
Payment #3 due on (or before) September 17, 2012