Occasionally you really need to pay the IRS on a single day without fail. This can happen in cases with real estate closings in which the IRS is a lien holder. The safest way to make this happen is a bank-to-bank wire transfer.
The IRS Same-Day Wire Federal Tax Payments at https://www.irs.gov/payments/same-day-wire-federal-tax-payments has a downloadable worksheet that you can fill out and take to your bank. There are of course fees involved.
There is one big caveat to keep in mind in using wire transfers. If your wire comes in after 5:00 PM ET, it will be returned to the bank.