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WASHINGTON (CBSMiami) — Time is running out if you haven’t filed your 2015 federal tax returns.

You have until midnight Monday to get them in the mail or file online.

Normally, the tax return deadline is April 15th but this year the Internal Revenue Service postponed it until Monday, April 18th thanks to a quirk of the calendar.

Emancipation Day is an official public holiday in the District of Columbia and it usually falls on April 16. The holiday celebrates the end of slavery. Since this year April 16th was a Saturday – not work day – then the observed Emancipation Day was moved to the previous day – Friday, April 15th.

Since Emancipation Day is a legal holiday, it gets precedence over the April 15 tax deadline.

“Section 7503 of the Code provides that, when April 15 falls on a Saturday, Sunday, or legal holiday, a return is considered timely filed if it is filed on the next succeeding day that is not a Saturday, Sunday, or legal holiday.”
IRS Revenue Ruling 2015-13

If you need more time to file, you can also use IRS Free File to get an automatic six-month extension to file your taxes. But if you owe taxes, those must be paid April 18th, even with an extension to file your return.

Believe it or not, taxpayers in Maine and Massachusetts have until tomorrow to file. That’s because April 18th is also the third Monday in April, which is the date that those two states observe Patriots’ Day. Therefore, their residents will have until Tuesday, April 19th, to file their 2015 tax returns.