In a museum in the city of Rome (in the year 2013) I read the following theory which is also found in the German language Wikipedia:

It is a known fact that very early Christians didn't celebrate Christmas but only other holidays - for example Easter.

When Christians started to celebrate Christmas, they were looking for a suitable date. They were choosing the date of a high Roman public holiday. And December 25th was a public holiday in Rome long before Christians started to celebrate Christmas on that day.

Because this day was a public holiday in Rome, they didn't have to go to work that day and because they didn't believe in the pagan religion, they didn't participate in the Roman ceremonies that were celebrated on that day. So they had time to celebrate the newly introduced Christian holiday.

And even more practical: In times of persecution nobody could use the fact that they did not celebrate on a pagan holiday as proof that they were Christians!

The fact that Christmas was initially only celebrated in the Roman empire on December 25th and Christians outside the Roman empire did not celebrate Christmas on December 25th makes this theory at least plausible.

In a museum in the city of Rome (in the year 2013) I read the following theory which is also found in the German language Wikipedia.

(However, according to the Wikipedia entry it is not sure if the theory really reflects the facts.)

It is a known fact that very early Christians didn't celebrate Christmas but only other holidays - for example Easter.

When Christians started to celebrate Christmas, they were looking for a suitable date. They were choosing the date of a high Roman public holiday. And December 25th was a public holiday in Rome long before Christians started to celebrate Christmas on that day.

Because this day was a public holiday in Rome, they didn't have to go to work that day and because they didn't believe in the pagan religion, they didn't participate in the Roman ceremonies that were celebrated on that day. So they had time to celebrate the newly introduced Christian holiday.

And even more practical: In times of persecution nobody could use the fact that they did not celebrate on a pagan holiday as proof that they were Christians!

The fact that Christmas was initially only celebrated in the Roman empire on December 25th and Christians outside the Roman empire did not celebrate Christmas on December 25th makes this theory at least plausible.