Estimated tax payments must be postmarked by the due date for the Internal Revenue Service to consider the payments made on time. For example, the first estimated payment is due on April 15, so as long as your envelope shows a postmark of April 15 or earlier, it's considered on time no matter when the IRS actually cashes the check.
Weekends and Holidays
If the due date for an estimated payment falls on a weekend or legal holiday, the payment is due the next business day. For example, if June 15, the second estimated tax payment deadline, falls on a Saturday, the payment is due June 17 because June 16 is a Sunday. As long as your payment is postmarked by the extended deadline, you're safe.
