Question
Why is the Jewish Sabbath (Shabbat) celebrated on Friday?
Answer
On the seventh day of creation, God rested from all the work of the previous six days of creation (See the Book of Genesis). The Torah tells the Jewish people that they also should rest on the seventh day.
In Judaism, days are counted from sunset on one day until sunset on the next day. So the seventh day of creation corresponds to sunset on Friday evening to sunset on Saturday evening.