When is Christmas celebrated, and why are there different dates?

Most of the world celebrates Christmas on December 25. But in many churches and countries you’ll also see Christmas on January 7, January 6, or (in one famous local exception) January 19. The differences aren’t about different stories—they’re mostly about different calendars and a few older traditions.

The quick answer

Dec 25 (Gregorian / Revised Julian)
Roman Catholics, most Protestants, and many Eastern Orthodox Churches (those using the Revised Julian calendar) celebrate on this date.
Jan 7 (Gregorian date of Dec 25 in the Julian/Coptic calendars)
Russian, Serbian, Georgian (many), Jerusalem Patriarchate, and monastic communities on Mount Athos that keep the old Julian calendar; also Coptic and Ethiopian Orthodox Churches.
Jan 6
The Armenian Apostolic Church (outside the Holy Land) keeps the older custom of celebrating the Nativity and Theophany together on January 6.
Jan 19 (local Armenian exception)
The Armenian Patriarchate of Jerusalem follows the Julian reckoning for fixed feasts in the Holy Land, so their January 6 falls on January 19 in the civil (Gregorian) calendar.

Why the dates differ

1) Calendar math

The Gregorian calendar (introduced in 1582) corrected the drift of the earlier Julian calendar by adjusting leap years.
Today, the Julian calendar runs 13 days behind the Gregorian. So Julian Dec 25 = Gregorian Jan 7.
Note for the future: in 2100 the gap becomes 14 days, so Julian Dec 25 will correspond to Gregorian Jan 8.

2) Which calendar a church uses

Gregorian for most Western churches.
Revised Julian (adopted by many Orthodox churches in the 20th century) matches the Gregorian for fixed feasts like Christmas until the year 2800.
Julian for several Orthodox churches that kept the older system.
Coptic and Ethiopian calendars are closely tied to the Julian for fixed feasts, which is why their Christmas also appears on Jan 7 in the civil calendar.

3) Different ancient customs

In the earliest centuries, some Christians celebrated Epiphany/Theophany (the manifestation of Christ) on January 6, including the Nativity within that feast.
Over time, the West separated the feasts, fixing Christmas on Dec 25 and Epiphany on Jan 6. The Armenian Church preserved the older combined celebration on Jan 6.

A short historical note

By the 4th century, Rome and many other churches had settled on December 25 for Christmas. The date likely connected with theological symbolism about light in darkness around the winter solstice and with the growing desire to celebrate the Incarnation distinctly from Epiphany. The exact historical birthday of Jesus isn’t known; the date is liturgical, not archival.

How this plays out around the world

Even within the same country, you may find two Christmas dates depending on the community’s church affiliation. In places with multiple traditions or large diasporas, many people happily mark both celebrations. Civil holidays usually follow the majority tradition, but church services keep the liturgical calendar of each church.

Cheat sheet

Dec 24 evening (Christmas Eve): Vigil and family gatherings in many Western traditions.
Dec 25: Christmas for Catholics, most Protestants, and many Orthodox on the Revised Julian.
Jan 6: Epiphany/Theophany in the West and many Eastern churches; Christmas + Theophany together for most Armenians.
Jan 7: Christmas for Julian-calendar Orthodox, plus Coptic and Ethiopian Orthodox.
Jan 19: Christmas/Theophany for the Armenian Patriarchate of Jerusalem (local calendar usage).

One last wrinkle: “the twelve days”

In Western Christianity, “the twelve days of Christmas” run Dec 25–Jan 5, ending with Epiphany (Jan 6). In Julian-keeping churches, the analogous festive period shifts accordingly into January.

Christmas is “on different days” mostly because churches keep time by different calendars. The story and the meaning are shared; the page of the calendar isn’t.