This holiday season features a rare calendar quirk only seen once since 1959
This week, two holidays are being celebrated across the country, as the first night of Hanukkah falls on the same day as Christmas.
The Jewish celebration of the Festival of Lights begins at sundown Wednesday, just as Christians are celebrating the birth of Jesus Christ.
A "Christmukkah" as rare as this one occurs due to the Hebrew calendar not aligning with the Gregorian calendar, causing the start of the Hanukkah holiday to move dates annually.
The holidays have occurred on the same day four times since 1900, according to Hebcal, a website that tracks Jewish holidays and the Hebrew calendar.
Here's what you need to know about Christmukkah and this year's rare calendar quirk.
What is Christmukkah?
Christmukkah – sometimes spelled Chrismukkah – is a combination celebration that occurs when Hanukkah is celebrated in conjunction with Christmas. Or, in a rare case, it marks the first day of Hanukkah occurring on the same day as Christmas.
The term is thought to have been in use for centuries, but a 2003 episode of "The OC" featuring a Christmukkah celebration added to its pop culture notoriety in the U.S.
While Christmukkah can be a lighthearted holiday to bring groups together, some have also acknowledged the commercialization of the fake holiday.
Have Christmas and the first day of Hanukkah coincided before?
The last time Christmas and the first day of Hanukkah occurred together was in 2005. Before that it occurred in 1959 and 1921.
When is the next time the first day of Hanukkah falls on Christmas?
Christmas and the first day of Hanukkah will line up in 2035, then again in 2054.
Why is a Christmukkah like this so rare?
Christmas, which commemorates the birth of Jesus Christ, is celebrated on the same day each year. Hanukkah recalls a Jewish revolt against Greek-Syrian oppressors more than 2,000 years ago and the legend of a miraculously long-lasting vial of oil. It also starts the same night each year – but on a different calendar.
Hanukkah is always observed on the 25th day of Kislev, the ninth month of the Hebrew calendar. That Hebrew calendar is based on a combination of lunar and solar cycles. But the Gregorian calendar established by Pope Gregory XIII in 1582 defines one year by the time it takes for Earth to travel around the sun.
The two systems don't line up, so Hanukkah drifts through the modern Gregorian calendar.