Difference in calendars
The main reason is the use of different calendars:

The Gregorian calendar, which was introduced in 1582 by Pope Gregory XIII, is considered more accurate from an astronomical point of view. This calendar replaced the Julian calendar, which had errors in calculating the length of the year.
The Julian calendar, developed back in 46 BC by decree of Julius Caesar, was used for centuries, but by the 16th century it began to "lag" behind the astronomical year by 10 days.
Most Western countries (Catholic and Protestant) have switched to the Gregorian calendar, so they celebrate Christmas on December 25. Orthodox churches in the CIS countries, however, continue to use the Julian calendar for liturgy. This shifts the date of the holiday by 13 days, resulting in the celebration on January 7.

Religious differences
Christmas is an important religious holiday for all Christians, but differences in traditions between Catholicism, Protestantism and Orthodoxy also play a role.

Catholic and Protestant churches adhere to the Gregorian calendar.
Most Orthodox churches, including the Russian, Serbian and Georgian, continue to follow the Julian calendar.
It is worth noting, however, that there are exceptions: for example, the Greek Orthodox Church celebrates Christmas on December 25.

Historical aspects in the CIS
After the October Revolution of 1917 in the Soviet Union, Christmas became a virtually forgotten holiday. It was excluded from the state calendar and for a long time was celebrated only among believers. While Western countries continued to actively celebrate December 25, the USSR focused on the New Year as the main winter holiday.

After the collapse of the Soviet Union, Christmas again became an official holiday, but traditions were preserved: Orthodox churches continued to follow the Julian calendar, which means celebrating Christmas on January 7.

Globalization and modernity
Nowadays, more and more people in the CIS countries take part in the celebration of December 25. Globalization, accessibility of information and the influence of Western culture contribute to the fact that many begin to decorate the Christmas tree, give gifts and feel the Christmas spirit already at the end of December.

However, for believers, January 7 remains the main date of Christmas, filled with special spiritual meaning.

Conclusion
Thus, the difference in the dates of celebrating Christmas is due to historical, cultural and religious reasons. However, regardless of the date, this holiday unites people all over the world, filling their homes with warmth, kindness and joy.