Today some ḥasidic communities, keeping alive a venerable Ashkenazi custom, refrain from studying Torah in the evening on December 24, usually staying home and playing chess or cards. Itzik Gottesman examines various explanations as to why:
Older Jewish religious texts instructed all Jews to stay home on Christmas Eve because Christians might attack or even kill them. Historically speaking, however, far more acts of violence were committed against Jews during Easter, when Christians mark the day Jesus died, than during Christmas, when he was born.
Over the centuries, these dangers generated a substantial folklore. Jews believed that on Christmas Eve, the Christian deity flew around and controlled the night. If any Jew were to open a Jewish holy text that night, that spirit could appear at any time and defile the holy book.
Jews also had fears and traditions surrounding the winter solstice, which falls a few days before Christmas. . . . Jews believed that on this night when the seasons changed, the earth was left unprotected. An old tradition connected to the winter solstice was to cover all pots that held well water so that the water would not be contaminated.
Much, but not all, of this changed when Jews came to America. Indeed, Gottesman notes, even the religiously conservative Yiddish newspaper Morgn Zhurnal published numerous advertisements from local businesses wishing their Jewish customers freylekhn (happy) Christmas.
Read more on Forward: https://forward.com/yiddish/436870/when-christmas-was-a-time-of-fear-for-jews/