This week marks a special occasion in the United States, as the first night of Hanukkah coincides with Christmas.

The Jewish Festival of Lights begins at sundown on Wednesday, aligning with the Christian celebration of Jesus Christ’s birth.

This particular overlap of “Christmukkah” happens infrequently because the Hebrew calendar does not consistently match the Gregorian calendar, resulting in Hanukkah dates shifting every year.

Since 1900, this alignment has occurred four times, as reported by Hebcal, a resource that monitors Jewish holidays and the Hebrew calendar.

Here’s everything you should know about this year’s unusual Christmukkah.

What does Christmukkah mean?

Christmukkah—sometimes spelled Chrismukkah—is a festive event that arises when Hanukkah is celebrated on the same day as Christmas. In rare instances, it signifies the first day of Hanukkah falling on Christmas.

The term has been around for centuries, but it gained popularity after a 2003 episode of “The OC” featured a Christmukkah celebration, boosting its recognition in U.S. culture.

While Christmukkah serves as a fun way to unite different groups, some voices have raised concerns about the commercialization of this hybrid holiday.

Have the first day of Hanukkah and Christmas ever shared the same date before?

The last occurrence of Christmas and the first night of Hanukkah falling on the same day was in 2005. Prior to that, it happened in 1959 and 1921.

When will the first day of Hanukkah and Christmas overlap again?

Why is this Christmukkah so unusual?

Christmas, which celebrates the birth of Jesus Christ, always falls on the same date every year. In contrast, Hanukkah recalls a significant Jewish uprising over 2,000 years ago and the miracle of a long-lasting oil supply. Although Hanukkah starts on the same night annually, it follows a different calendar system.

Hanukkah is always observed on the 25th of Kislev, the ninth month of the Hebrew calendar. This calendar is based on both lunar and solar cycles, while the Gregorian calendar—established by Pope Gregory XIII in 1582—defines a year based on the Earth’s orbit around the sun.

Because these two calendars don’t synchronize perfectly, Hanukkah dates shift over time within the Gregorian calendar.