We all know the expression “once in a blue moon,” or any number of figures of speech involving rare celestial events. Well, allow us: Every couple of years, the stars perfectly align, and Tax Day gets pushed back a few days.

Such is the case this year. The federal tax deadline, typically on April 15, this year falls on April 18. And for the lucky residents of two states, taxes aren’t due until April 19. The extra cushion has nothing to do with the pandemic, which pushed tax filing deadlines back considerably in each of the past two years. This year’s extension has to do with the clash of the traditional April 15 Tax Day with two lesser-known local holidays.

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Why is Tax Day on April 18?

The answer is simple. The deadline to file federal taxes is on Monday, April 18, this year because the Internal Revenue Service’s District of Columbia offices will be closed on Friday, April 15, in observance of the locally recognized Emancipation Day.

The D.C. public holiday technically falls on Saturday, April 16, but is observed by the government on the closest weekday when it falls on a weekend.

Congress initially set the filing deadline in March, but it revised the Internal Revenue Code in 1954, moving the deadline to April 15, where it remains.

But through 67 years, Tax Day has fallen outside of April 15 26 times, according to Timeandate.com. In most cases, the deadline got pushed back to the 16th or 17th simply because the 15th fell on a weekend. Since DC Emancipation Day came officially online in 2005, Tax Day has landed on April 15 only seven times, the last time being in 2019. Emancipation Day will postpone the IRS deadline again in 2023.

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Another regional holiday grants the residents of Massachusetts and Maine an additional day to report their taxes.

Patriots’ Day is observed on the third Monday each April in the two New England states. This year, that lands on April 18, meaning filers in those states have until Tuesday the 19th to file. Many Bostonians will be cheering on the runners at the Boston Marathon while much of the rest of the country labors away sorting out their tax documents to meet the Monday deadline.