Nittel Nacht is a name given to Christmas Eve by Jewish scholars in the 17th century, although Rabbi Samuel Eidels already observed the day by the late 16th century. In the Middle Ages (in Christendom), Jews were forbidden from appearing in public during the high Christmas holidays, and as such the day marked the beginning of a siege of sorts for certain Jewish populations. Jewish mystics believed apostates were conceived on the day and as a result Rabbis forbade married couples from sex on Nittel Nacht. Studying the Torah was also forbidden, although some read the Toledot Yeshu instead. Passing the time playing card games or chess was also popular.
After the advent of the Gregorian Calendar, Orthodox Christians and Catholic Christians observed Christmas Eve on two separate dates; this led to Rabbinic debate, and Nittel Nacht is observed in accordance with the local Christian community. Certain pious Jews observed Nittel Nacht twice each year.
In modern times, with less tense Judeo-Christian relations, Nittel Nacht is less observed, although certain Orthodox denominations still observe it.