Listen up, because this is a weird coincidence that you’re definitely going to have to factor in to your holiday plans: Because of the lunar calendar, 9/11 will fall on Christmas Eve this year.

Needless to say, this overlap is one for the books. It’s been hundreds of years since the last time 9/11 fell on Christmas Eve, and it’ll be another couple hundred before it ever happens again. Unfortunately, this once-in-a-lifetime coincidence does mean that your holiday cheer will be getting a little dose of somber remembrance this year. But the reason why is actually pretty interesting.

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The uncommon occurrence can be attributed to the lunar calendar, which has a longer cycle than the standard Gregorian calendar that governs holidays like Christmas. And no one alive has experienced a day quite like it—the last time 9/11 fell on Christmas Eve was in 1714, though it did coincide with Christmas Day itself just 16 years later in 1730.

Christmas Eve is typically a time for family and celebration, but this year, you’ll definitely need to leave some time between drinking eggnog and wrapping presents for quietly reflecting on the lives that were lost on that fateful day when terrorists attacked our nation. So be sure to plan accordingly!

Experts have urged families traveling for the Christmas holidays to expect delays because of the unusual way 9/11 has fallen on the calendar this year. However, it’s worth noting that Christmas Eve and 9/11 will, of course, not sync up in areas that employ other calendar systems—in much of Latin America, for example, 9/11 is held on December 3.

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Wow. What an interesting coincidence. Looks like Christmas Eve and 9/11 will be especially memorable this year!