If you file quarterly estimated tax payments, your Q2 payment should be mailed by Thursday.

If you mail your estimated tax payment and the date of the U.S. postmark is on or before the due date, the IRS will generally consider the payment to be on time. If you use IRS Direct Pay, you can make payments up to 8 p.m. Eastern time on the due date. If you use a credit or a debit card, you can make payments up to midnight on the due date. [SOURCE]

If you don’t pay enough tax through withholding and estimated tax payments, you may be charged a penalty. You also may be charged a penalty if your estimated tax payments are late, even if you are due a refund when you file your tax return. [SOURCE]