Here’s a seemingly simple question: When is Tax Day this year?

Even for the most tax-law-challenged taxpayers, the answer might seem obvious: Individual income-tax returns are due April 15, a date as memorable to most Americans as the joyful holidays of Jan. 1, July 4 or Dec. 25.

Simple but wrong—for this year and last year and next year, for that matter.

When it comes to taxes, it seems that rarely is anything truly simple, even Tax Day. The deadline can vary depending on several factors, such as the timing of weekends and the celebration of Emancipation Day in the District of Columbia. The date also may vary for millions of people, depending on various state holidays and even the timing of natural disasters such as severe storms and tornadoes.

So for most taxpayers this year, T-Day falls on April 18.

Here is a mercifully brief explanation:

April 15 this year falls on a Saturday. So it might be logical to assume that would push the tax-filing deadline back to Monday the 17th. No, because that’s the day that Washington, D.C., will observe Emancipation Day this year.

That refers to emancipation from slavery, not taxes: the celebration of when President Abraham Lincoln signed a law on April 16, 1862, to end slavery in Washington. This year, the 16th is a Sunday, making Monday the 17th the day on which Emancipation Day is observed—and bumping back the filing deadline for most people to Tuesday the 18th.

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“By law, D.C. holidays impact tax deadlines for everyone in the same way federal holidays do,” the IRS explained earlier this year. Thus, the April deadline for filing and paying whatever might be owed to Uncle Sam for the 2016 tax year is Tuesday, April 18—at least for most taxpayers.

But not all.

Many people in parts of Georgia and Mississippi have until May 31 to file and pay, and parts of Louisiana get until June 30, an IRS spokesman says. There are other exceptions, too—as well as different deadlines in some states.

The IRS can postpone certain deadlines for taxpayers who live, or have a business, in places designated as a federal disaster area. That’s why some parts of Georgia, Mississippi and Louisiana have been given later filing deadlines this year. (For details, see the IRS website: irs.gov and look under “Tax Relief in Disaster Situations.”) There are other deadlines for many taxpayers living and working abroad, as well as for members of the military serving in a combat zone.

For millions of procrastinators, of course, the April filing deadline simply means the deadline for requesting more time to file. They can ask the IRS for an additional six months, says Jeffery L. Yablon, a partner at Pillsbury Winthrop Shaw Pittman LLP in Washington who has compiled a splendid collection of quotes, quips and pithy sayings about taxes. (Here is one example: “Taxes: Of life’s two certainties, the only one for which you can get an automatic extension.” Mr. Yablon attributes that to “anonymous,” but reliable sources say it comes from a book called “Buzzwords” by John Freund and David Porter.)

To get a six-month filing extension, you don’t even need to give a reason. But that doesn’t give you more time to pay whatever you might owe. It only gives you more time to file. However, a six-month extension won’t extend the deadline until Oct. 18. Instead, it will extend it to Monday, Oct. 16, the IRS says.

The IRS hasn’t officially set the filing date yet for 2018, but it definitely won’t be April 15 since that will fall on a Sunday. While it’s hazardous to make any predictions about subjects involving taxes, especially this year, reliable sources say, based on the latest IRS guidance, T-Day 2018 will fall on April 17—thus marking the third year in a row that the dreaded date won’t be April 15.

The filing date in 2016 was tricky, too. Here’s how the IRS explained it a year ago: “Taxpayers have until Monday, April 18 to file their 2015 tax returns and pay any tax due because of the Emancipation Day holiday in Washington, D.C., falling on Friday, April 15. Taxpayers in Maine and Massachusetts will have until Tuesday, April 19 because of Patriots’ Day observances on April 18.”

While many states follow the federal custom, some don’t. When in doubt, check with your state tax department.

Why not scrap the April T-Day madness? Accountants, enrolled agents and other tax preparers have complained for years about the annual stresses of the April filing deadline, especially since so many perplexed clients wait until the 11th hour to share their shoeboxes stuffed with W-2 forms, 1099s, charitable-giving receipts and stacks of other documents.

Some people have suggested moving the deadline to Election Day—or moving Election Day to April—so that voters will focus more closely on what they get compared with what they pay.

However, to change the date, congressional action would be required. And the path to congressional action on most tax-related subjects may be one of the few areas at least as complicated as our tax laws themselves.

Mr. Herman is a writer in New York and formerly The Wall Street Journal’s Tax Report columnist. Email him at taxquestions@wsj.com.

Appeared in the Mar. 06, 2017, print edition as 'Why April 15 Has Lost Its Claim as ‘Tax Day’.'

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