In this photo, an Illinois Department of Revenue employee offers assistance to income tax payers at the Illinois Department of Revenue, in Springfield, Ill. Even with the advent of electronic filing, many Americans may not be able to deliver their tax return before the deadline Monday, April 15, 2013, at 11:59 p.m. EDT.

This weekend may not be very relaxing for millions of taxpayers. That's because they're beginning the last-minute scramble to file their returns on Tax Day.

Even with the advent of electronic filing, many Americans may not be able to deliver their tax return before the Monday, 11:59 p.m. EDT deadline.

If you're in this group, it may be OK. The IRS will give you until Oct. 15 to file your return if you ask for an extension by midnight Monday. Last year, 10.7 million Americans did just that.

But an extension won't buy you more time if you owe Uncle Sam any federal income taxes. And failing to pay by Monday's deadline means you will incur interest charges and fees on top of what you owe.

"Your extension is not extending your payment time," says Jackie Perlman, principal tax analyst at the research arm of H&R Block.

Still scrambling? Here are some guidelines for how to go about filing for an extension on your tax return and address a late filing if you owe income tax:

HOW TO FILE

Tax returns are due by midnight on Monday. If you're going to miss the deadline, file a request for an extension with the Internal Revenue Service. The IRS accepts extension requests several ways: By mail, via tax-preparation software, through a paid tax preparer, or online via the IRS website, www.irs.gov .

SELECT THE RIGHT FORM

To request the extension, you must fill out IRS Form 4868, which is available on the IRS website, through most tax preparation software, as well as in public libraries and post offices.

If you opt to mail in the form it has to be postmarked by Monday to be considered on time. Forms filed online on the IRS website or by using tax software can be sent in as late as 11:59 p.m. EDT on Monday. One bonus of filing online is the IRS will confirm receipt of the request form right away.

ESTIMATE HOW MUCH YOU OWE

When you fill out the form, the IRS requires that you estimate how much income taxes you owe and pay that amount.

If you file for an extension online, you can also pay what you owe through an electronic funds withdrawal from a checking or savings account.

The IRS also accepts credit and debit cards, as well as checks and money orders.

PAY UP, AVOID PENALTIES

Failing to pay your tax bill by the Monday deadline means you'll incur a slew of interest and penalty charges on what you owe.

The IRS assesses a monthly penalty of 0.5 percent of the balance owed, plus interest of 3 percent per year, compounded daily.

The last thing you want to do is miss the tax filing deadline and not ask for an extension. That triggers a penalty of 5 percent of the tax owed for each month the tax return is late.

"If it turns out you have a big balance due, don't make the mistake of not filing," Perlman says. "That's not going to help you."

CONSIDER A PAYMENT AGREEMENT

If you can't pay the full amount you owe on time, you may be able to set up a payment plan with the IRS.

Taxpayers who owe $50,000 or less, including penalties and interest, can arrange to pay on a monthly basis for up to 72 months. You can elect to do this before receiving a late-payment notice from the IRS.

You'll need to fill out and mail IRS Form 9465, which can be downloaded on the IRS website, along with your tax return.

The IRS also may determine you're eligible for an "offer-in-compromise," which essentially allows taxpayers to pay less than what they owe.

To figure out if you're eligible, use the pre-qualifier tool on the IRS website.

REMEMBER YOU ONLY GET 6 MONTHS EXTRA

Once the IRS accepts your extension request, you'll have until Oct. 15 to file a complete tax return. The IRS won't give any more time, says Perlman.

"That is the final, final (deadline)," she says. "If you file after that, your return is considered delinquent."

WHAT IF YOU'RE DUE A REFUND?

If you've calculated that you're due a refund but won't be able to file your return on time, it's not necessary to file for an extension.

"The extension, for the most part, is for folks that owe taxes and need to gather their paperwork," says Anabel Marquez, a spokeswoman for the IRS. "There won't be any penalties for not filing if a refund is owed."

Just because you don't have to stick to Monday's deadline doesn't mean Uncle Sam will hold on to your cash forever, though. Tax refunds that are not claimed within three years become property of the U.S. Treasury.