Why do atheists celebrate Christmas?

Perhaps the real question should be: Why do Christians celebrate Jesus' birth on the same day that Pagans celebrate(d) the shortest day of the year? For those of you that say, that the winter solstice is days before December 25th, let me give you a history and astronomy lesson.

Before 46BC, Pagans celebrated on the shortest day of the year. With all sorts of screwy calendars, they sometimes missed by a day or so. Then along came Julius Caesar who set the calendar at 365 and one quarter days, with the vernal equinox falling on the 25th of March (which made the winter solstice fall on the 25th of December). Now the Pagans knew when to celebrate the winter solstice. The Julian calendar was far better than anything that preceded it. However, the real time between vernal equinoxes is 11 minutes shorter than 365 and 1/4 days. This causes an error of one day every 128 years. The die was cast that Pagans celebrated on the 25th of December even as the calendar slowly drifted. By the fourth century, the winter solstice had drifted back to December 21st.

Wikipedia: There are different hypotheses regarding the date of Jesus's birth, and in the early fourth century, the church fixed the date as December 25. This corresponds to the traditional date of the winter solstice on the Roman calendar.

Like the Pagans sticking to December 25th, the Church is sticking with it as well. Biblical scholars pretty much agree (today) that Jesus was born in the spring.

To bring you further up to date, by 1582 the Catholic Church had had enough calendar drifting. The vernal equinox had drifted back to about March 11th. Pope Gregory XIII shortened the year by eliminating three leap days every 400 years. At the same time, he eliminated ten days in 1582 so the day after 04-OCT-1582 was 15-OCT-1582. This set the date of the vernal equinox back to March 21st, the same day that it was back during the First Council of Nicea. Gradually, just about everyone has now shifted to the Gregorian calendar. The USA shifted in 1752 by dropping 11 days (1700 was not a leap year in the Gregorian calendar).

While yes, His Passion, Death, and Resurrection are what mattered, the fact that He Was Born meant  that that could happen.

In biblical times, it didn't matter

So that's why it took a few centuries to decide on when he was born.....

Many scholars do not see the Luke and Matthew nativity stories as historically factual, regarding them as laced with theology and presenting two substantially different accounts and genealogies. For instance, they point to Matthew's account of the appearance of an angel to Joseph in a dream; the wise men from the East; the massacre of the innocents; and the flight to Egypt, which do not appear in Luke, which instead describes the appearance of an angel to Mary; the Roman census; the birth in a manger; and the choir of angels appearing to the shepherds in the fields. 

Except they really, really messed up and instead of going for historical accuracy, the Catholics played politics and pandered to the pagans to get them to get right with Jesus.

Why wouldn't they? Christmas has evolved far beyond the original religious purpose.

Because it's not really about Jesus. It's just a holiday. Even people that at least sort of believe still don't typically celebrate Christmas for Jesus but for the holiday. It's about family, gifts, and decorations. Jesus is just an excuse for the holiday existing.

What does a decorated pine tree or gifts have to do with Jesus being born? Nothing. What does Santa Claus have to do with Jesus? Nothing. It's all holiday fluff.

Ever wonder what a bunny, wicker basket, painted eggs, and flowery decorations have to do with Jesus? Makes no sense, right? That's because Easter isn't about Jesus. It's a spring renewal and fertility celebration named after a pagan goddess by the same name, for whom a rabbit was a common representation. When Vikings intermingled with England, there was a mixing of cultures, languages, and customs, and Easter got lazily repurposed as about Jesus without really changing anything.

Hypocritical isnt it?