Your first estimated tax payment is usually due the 15th of April. You may pay the entire year's estimated tax at that time, or you may pay your estimated tax in four payments. The four payments are due April 15th, June 15th, September 15, and January 15th 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 is not a Saturday, Sunday, or legal holiday.

References:

• Publication 505, Tax Withholding and Estimated Tax, Chapter 2.

Source: www.irs.gov