ASKING THE IMPORTANT QUESTIONS: Hanukkah overlaps with Christmas this year. But why all the moving around? (Josh Hafner , USA TODAY).
Christmas (as celebrated in the Western Church) is timed according to the Gregorian calendar, which is a solar calendar. But Hanukkah is timed according to the Jewish religious calendar, which is technically a luni-solar calendar (not a lunar calendar, as the article says - although it does describe the details correctly). This Jewish calendar is based on the lunar cycle, but is periodically corrected with a leap month so that it stays in reasonably close alignment with the solar year. So the two holiday can be quite out of sync in their timing, but they do line up sometimes.
The reason many Orthodox Christian traditions celebrate Christmas on 7 January is that they are still marking it according to the old Julian calendar.
In antiquity the Enochic and Qumran Jews used a purely solar calendar that was rounded off to a 364 day year, which conveniently meant that each religious festival was on the same day of the week each year. But it also meant that their calendar moved out of alignment with the actual solar year by a day and a half each year. Whether and how this was corrected is not perfectly clear.
Past posts on Jewish calendars are here and here and links. Cross-file under 'Tis the Season and ditto (Hanukkah Edition).