Didn’t finish your 2015 income taxes last spring? If you filed a six-month extension and promptly forgot about it, it’s time to call your accountant. As MONEY points out, the final deadline to hand over your cash to the government is next Monday, October 17. File any later than that and you’ll face a penalty.

Scrambling to prepare your returns? You’re not alone: This past year, 13.5 million Americans were expected to request an extension. These tardy taxpayers had to cough up an estimate of what they owed for 2015 by April 18 (or in Maine and Massachusetts, April 19) and make plans to turn in the full amount by mid-October.

If you didn't pay an estimate when you requested your extension, expect to be slapped with some pretty hefty fines: a fee equal to 0.5 percent of the tax you initially owed per month you didn’t pay (up to 25 percent), to be precise. And if you continued to ignore those pesky letters from the IRS, this fee will rise to 1 percent per month. You’ll also have to pay interest on the money you haven’t submitted—a whopping 3 percent—along with the federal short-term rate, compounded each day.

Even if you paid the government 90 percent of what you thought you owed last April, you still have to submit all your tax materials next week. Fail to do so, and you’ll incur penalties for late filing and late payment, plus interest, USA Today points out.

Keep in mind that some people—like military members stationed in combat zones, or citizens in federally declared disaster areas—are granted additional time to file their taxes if they need it. And if you’ve experienced a non-federally declared disaster, you can file for an extension and the IRS will determine your eligibility on a case-by-case basis. (For additional details, visit the IRS website.)

If you’re scheduling an emergency session with your accountant now (or filling out all the forms yourself), don’t panic. Instead, read our list of tax-filing tips and see if you qualify for any last-minute deductions.

[h/t MONEY]

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